Support reading the DEM and restart files in netCDF format

[MSallermann]

Our netCDF class supports parsing the files (albeit in the very limited format that we are using so far).
It should be possible to read the original DEM from netCDF as well.

Since netCDF is a very flexible format, we would probably only be able to support very specific file structures. In the long term we could make use of a specialized library like GDAL.