Kevin Sahr
Add a page linking to software including `dggridR` and `pydggrid`.
DgHexC3Grid2D and DgHexC2Grid2D are similar; move the shared functionality into a base class for 2D grids with a substrate and surrogate grid. addFromSeq in DgHexC3Grid2D only works for Class I/III...
Consider making shapelib a module as per @r-barnes previous pull request.
DGGRID uses a very old version of clipper for polygon intersection. Replace this with the new C++ library [clipper2](https://www.angusj.com/clipper2/Docs/Overview.htm).
Remove the attempt for str2add (in all classes) to handle the case where the address is NULL. Not needed and too many potential issues.