Make NetCDF dependency optional

It could be a good idea to make the NetCDF optional but enable it by default.

We would need to disable all modules depending on netcdf. These are:

• mo_netcdf
• mo_ncwrite
• mo_ncread

mo_mcmc is depending on mo_ncwrite for writing out log-files. We should add a pre-processor flag to disable log-file support, when forces was compiled without netcdf support. If a log-file is specified in that case, an error should be thrown.