[fix] Make GeoJSON output valid by transforming to WGS84

[StefanBrand]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #188.

Description of Changes

If the database column has a SRID other than 4326 (WGS84), the geometry and---if configured---its bbox is transformed to 4326 to conform with the GeoJSON spec.

Questions

• TBD: Default value of the transform parameter. Should it default to None to keep the current behaviour?
• I think that currently the bbox of the feature would not be transformed because the extent is directly accessed. Maybe we need to move the functionality into get_attribute. -> out of scope
  

  django-rest-framework-gis/rest_framework_gis/serializers.py

  Lines 147 to 154 in da5acef

  	if self.Meta.auto_bbox and geo_value:
  	feature["bbox"] = geo_value.extent
  	# otherwise it can be determined via another field
  	elif self.Meta.bbox_geo_field:
  	field = self.fields[self.Meta.bbox_geo_field]
  	value = field.get_attribute(instance)
  	feature["bbox"] = value.extent if hasattr(value, 'extent') else None
  	processed_fields.add(self.Meta.bbox_geo_field)

[StefanBrand]

@auvipy thank you for your review. I have noted two questions of mine at the top. What are your thoughts on these?

[auvipy]

Answers:

1. To keep BC, Keeping None would be better choice I believe.

2. A new function / method might be better in this case, so we can give that a try

[StefanBrand]

I've added tests for the bbox of the geometry (which is correctly transformed) and decided that the bbox of the Feature is out of scope.

[auvipy]

"If any other" this line is not clear, can you please rephrase it?

[StefanBrand]

I have rephrased these docs after setting the default value to None. I hope it is easier to understand now. What do you think?