Flag to reduce precision of GeometryField

DRF 3.8 support

Fix default widget for GeometryFilter

Using `forms.BaseGeometryWidget` will cause a IsADirectoryError, because
of an empty template_name being used.
This was added by me in eb54fc06.

This patch changes it to use a simple TextInput; I can imagine using a
nicer widget here, but that requires to also add/handle the required
media assets, which was not trivial (IIRC).

```
Template error:
In template …/project/.venv/lib/python3.6/site-packages/django_filters/templates/django_filters/rest_framework/form.html, error at line 4
   21
   1 : {% load i18n %}
   2 : <h2>{% trans "Field filters" %}</h2>
   3 : <form class="form" action="" method="get">
   4 :      {{ filter.form.as_p }}
   5 :     <button type="submit" class="btn btn-primary">{% trans "Submit" %}</button>
   6 : </form>
   7 :

Traceback:

File "…/Vcs/django/django/core/handlers/exception.py" in inner
  35.             response = get_response(request)

File "…/Vcs/django/django/core/handlers/base.py" in _get_response
  158.                 response = self.process_exception_by_middleware(e, request)

File "…/Vcs/django/django/core/handlers/base.py" in _get_response
  156.                 response = response.render()

File "…/Vcs/django/django/template/response.py" in render
  106.             self.content = self.rendered_content

File "…/Vcs/django-rest-framework/rest_framework/response.py" in rendered_content
  72.         ret = renderer.render(self.data, accepted_media_type, context)

File "…/Vcs/django-rest-framework/rest_framework/renderers.py" in render
  724.         context = self.get_context(data, accepted_media_type, renderer_context)

File "…/Vcs/django-rest-framework/rest_framework/renderers.py" in get_context
  701.             'filter_form': self.get_filter_form(data, view, request),

File "…/Vcs/django-rest-framework/rest_framework/renderers.py" in get_filter_form
  635.                 html = backend().to_html(request, queryset, view)

File "…/project/.venv/lib/python3.6/site-packages/django_filters/rest_framework/backends.py" in to_html
  69.         return template.render(context, request)

File "…/Vcs/django/django/template/backends/django.py" in render
  61.             return self.template.render(context)

File "…/Vcs/django/django/template/base.py" in render
  175.                     return self._render(context)

File "…/Vcs/django/django/test/utils.py" in instrumented_test_render
  98.     return self.nodelist.render(context)

File "…/Vcs/django/django/template/base.py" in render
  943.                 bit = node.render_annotated(context)

File "…/Vcs/django/django/template/base.py" in render_annotated
  910.             return self.render(context)

File "…/Vcs/django/django/template/base.py" in render
  993.             output = self.filter_expression.resolve(context)

File "…/Vcs/django/django/template/base.py" in resolve
  676.                 obj = self.var.resolve(context)

File "…/Vcs/django/django/template/base.py" in resolve
  802.             value = self._resolve_lookup(context)

File "…/Vcs/django/django/template/base.py" in _resolve_lookup
  864.                             current = current()

File "…/Vcs/django/django/forms/forms.py" in as_p
  297.             errors_on_separate_row=True)

File "…/Vcs/django/django/forms/forms.py" in _html_output
  238.                     'field_name': bf.html_name,

File "…/Vcs/django/django/utils/html.py" in <lambda>
  371.     klass.__str__ = lambda self: mark_safe(klass_str(self))

File "…/Vcs/django/django/forms/boundfield.py" in __str__
  36.         return self.as_widget()

File "…/Vcs/django/django/forms/boundfield.py" in as_widget
  118.             **kwargs

File "…/Vcs/django/django/forms/widgets.py" in render
  235.         return self._render(self.template_name, context, renderer)

File "…/Vcs/django/django/forms/widgets.py" in _render
  240.         return mark_safe(renderer.render(template_name, context))

File "…/Vcs/django/django/forms/renderers.py" in render
  30.         template = self.get_template(template_name)

File "…/Vcs/django/django/forms/renderers.py" in get_template
  36.         return self.engine.get_template(template_name)

File "…/Vcs/django/django/template/backends/django.py" in get_template
  34.             return Template(self.engine.get_template(template_name), self)

File "…/Vcs/django/django/template/engine.py" in get_template
  144.         template, origin = self.find_template(template_name)

File "…/Vcs/django/django/template/engine.py" in find_template
  126.                 template = loader.get_template(name, skip=skip)

File "…/Vcs/django/django/template/loaders/base.py" in get_template
  24.                 contents = self.get_contents(origin)

File "…/Vcs/django/django/template/loaders/filesystem.py" in get_contents
  23.             with open(origin.name, encoding=self.engine.file_charset) as fp:

Exception Type: IsADirectoryError at /api/foo/
Exception Value: [Errno 21] Is a directory: '…/Vcs/django/django/forms/templates'
```

Make ValidationError more user-friendly

removing every reference of olwidget & making admin widget as default geodjango widget.

[QA] Changed django test requirement from 2.0b1 to 2.0

[QA] Change django-filter test requirement to latest

Bring support for django 2.0 and drf 3.7

along the way drop django1.9 and 1.10

Corrected spelling mistake in README

Update README.rst

Latitude and longitude are documented in the wrong order for in_bbox filter.

Added support for GeometryCollection fields

Fixes for test_geojson_performance with DRF >=3.5

The test_geojson_performance test was failing an assertion in DRF>=3.5.  Fixed by adding fields='__all__'

Fixes for Django 1.11

Changed in https://github.com/django/django/commit/50613d957a.

The default widget for `GeometryField` (`OpenLayersWidget`) was changed
to use `map_srid = 3857` in https://github.com/django/django/commit/93eca976.

[RFC] add tox.ini / support for pytest