compile BENCH without any NetCDF library

Context

To facilitate tests/benchmarks on new/experimental machines it can be useful to be able to compile and run NEMO without the need of the NetCDF library (of course in this case we must not request any input/output in NEMO).

Proposal

• add in the ext directory a DUMMY_NETCDF directory that will contain all the netcdf interface with dummy routines so, if we use them the model can compile. This routines are the empty routines based on the original netcdf_fortran routines and always returning -1 so the model will stop if we try to use them.
• modify makenemo to add new options, such as --nocdf so that we use this external dummy NetCDF insetad of the default library.