Add GEF file support

[JulesBlm]

GEF (Geotechnical Exchange Format) is an ASCII based file format commonly used in the Netherlands for CPT and borehole data. It is used as input the Deltares D-series software like D-Foundations. A lot of data in DINOloket and BROloket is the GEF format.

GEF boreholes can be inclined.

Prior Art

• pygef. A library that makes use of polars for dataframes and uses the Rust package gef-file-to-map for parsing the GEF files. While polars dataframe to pandas dataframe conversion is trivial, we should consider wether having polars as a dependency alongside pandas is desirable. Also supports (IMBRO)XML-based formats for soil measurements but I don't know much about that yet.
• Groundhog, a general-purpose geotechnical library uses regular expressions to parse data from a GEF file.

[JulesBlm]

Would be nice to infer the CRSs used from the #XYID and #ZID header keywords to automatically use the correct CRS for transforming the geospatial data.