Use Geopandas for geocoding to LSOA and GSP

[JamieTaylor-TUOS]

When geocoding using boundaries, use Geopandas.

Linked to #5

When Geocode lib was first authored, Geopandas was proving difficult to install due to version conflicts with other Python libs and missing C binaries on Windows. This now appears to be resolved, but also dev should prioritise Linux operating systems (ideally containerised).

[Herald-TUOS]

Shall I assign this to me?

[JamieTaylor-TUOS]

If you have capacity 👍🏻

[Herald-TUOS]

We decided to use Geopandas throughout the module. Make changes to return GeoDataFrame from ONS_NRS._load_llsoa_boundaries and NationalGrid.load_gsp_boundaries methods.