Make geospatial type configurable

[svniemeijer]

At the moment the postgis backend always uses 'geography' and spatialite always 'geometry'.

For spatialite there is no other option but 'geometry', but for postgis it would be possible to support 'geometry' instead of 'geography'. Deployments that require a geometry representation would benefit from having an option in muninn to allow this to be specified.

In addition, such a configuration option could then also be used to allow completely disabling geospatial data types. This would allow muninn to be used with postgresql without postgis and sqlite without spatialite. This will make deployments easier for cases where geospatial information is not applicable.

[svniemeijer]

We should potentially also add functions that allow to translate between geography types and geometry types.
Going from geography to geometry involves splitting polygons/lines at the datelines and to add points for polygons that cover a pole (to have the polygon run along the bottom/top edge of the 2D plane).