Handle WKT Extension Tag

[FObermaier]

EPSG.IO nowadays offers WKT for spatial reference systems that may contain an EXTENSION tag.
This leads to parsing exceptions. A first step would be to quietly disregard it, but not fail.
see: stackoverflow

[FObermaier]

A second step could be to integrate https://github.com/NetTopologySuite/ProjNet4GeoAPI/tree/enhancement/grid_support and wire it.