Greenland datasets: add support for gridded dH/dt observations

Greenland datasets: Currently, dH/dt observerations, needed for optimization / initialization, are added as a separate step (partly because of projection issues). In the long terms, we'd like to add native support here for including them. The relevant data are here (currently for Greenland only) in the NERSC project directories: /project/projectdirs/piscees/data/gis/dhdt-Csatho
