RNetCDF release 2.0-3
- Support NetCDF-4 features for reading and writing of datasets.
- Suggest bit64 package for NC_INT64 and NC_UINT64 types.
- Drop support for netcdf-3.x library, require netcdf-4.x.
- Drop support for udunits-1 library.
- Disable calendar functions if udunits-2.x is not found during build.