pyproj.CRS object.
Returns None if the CRS is not set, and to set the value it
:getter: Returns a pyproj.CRS or None. When setting, the value
can be anything accepted by
pyproj.CRS.from_user_input() <pyproj.crs.CRS.from_user_input>,
such as an authority string (eg “EPSG:4326”) or a WKT string.
Note: This assumes all records in the GeoSeries are assumed to have the same CRS.

