Add KML export

[nemesifier]

KML export like in the old 0.9.x is still missing.

Here it is how it was implemented in the old version:

• view: https://github.com/ninuxorg/nodeshot/blob/0.9.x/nodeshot/views.py#L471-L498
• template: https://github.com/ninuxorg/nodeshot/blob/0.9.x/nodeshot/templates/kml.xml

The current version is a bit more complex before some information (like colors) are dynamic and must be fetched from the database.

[claudyus]

I don't know which requirement lead to develop a KML export feature in the previous version of nodeshot but a GPX exporter is desirable IMO because:

• it is not rely on closed product like google earth
• KML and KMZ can be generated by GPX file
• GPX is the facto standard and supported by almost any gis program (google earth included)
• QGis support GPX
• library are available to create GPX file in python https://github.com/tkrajina/gpxpy

[nemesifier]

We likely have to support several different formats. We support GeoJSON, we'll have to support GeoRSS for backward compatibility with OpenWISP, so it would be nice to support as many as we can.

I wonder if how much work would take to add a GPX serializer in https://github.com/djangonauts/django-rest-framework-gis that's the best place where these features should reside, it's all a matter of time, but hopefully other contributors of DRF-gis can help out.