Allow tags to accept geo fields

[thclark]

Feature request

Use Case

It's frequent to want to tag fields with geometric attributes, like a boundary polygon or a position. Being able to accept GeoJSON entries as tags would be a significant advantage since we could attach geospatial metadata to files.

A key use case is in https://github.com/windpioneers/windquest/pull/829 where we need to be able to do postGIS based filtering operations on datafiles.

Current state

It's possible to make latitude and longitude float tags, but this won't create a geo field in django.

Proposed steps

Use a serialiser to either take the raw string WKT value of the geometry (more compact serialisation), or serialise the geometry as geojson (probably more easily portable), within the tag serialisation methods.