Add to_geopandas method for BUFR data

[sandorkertesz]

This will be the first implementation of a to_geopandas() method in earthkit-data. Things to consider:

• BUFR data extraction is based onpdbufr, which can extract the geometry and crs columns required by geopandas. So the conversion should be straightforward.
• geopandas will bring in new (binary) dependencies, they should be optional