Added basic netCDF library support

Hi M.Lund:  Thank you so much for pulling xmgr into the 64 bit world.  I support one user who cannot live without it and I've been struggling to keep supporting xmgr for years.  We need netCDF, so here is a commit with minimal netCDF support.  I'm new to cmake, so please let me know if this makes sense and consider adding it to your branch.

Thanks much!

--Doug Hunt