Add means of converting coordinates in `Gazetteer.Place` for `geom` field

[griff-rees]

All the Gazetteer models have geom fields. In the current configuration of alto2txt2fixture.create_adjacent_tables json and csv results, Place includes longitude and latitude fields. These can be combined into a geom field when imported into Gazetteer.Place.

Tasks:

• Check if there is any advantage in implementing this within alto2txt2fixture (reason not is requiring geopandas...)
• Add as a func within lwmdb.utils to convert those fields into a GeometryCollection
• Include a way to check the srid
• Add basic doctest for that func independent of Gazetteer.Place (for example see the download_file docstring which auto renders and is included in pytest)
• Add a method on func to add these and a test within gazetteer.tests