Use a wrapper for some netcdf calls in gs2_io

Wrap calls to nf90_def_var and nf90_put_att to handle creation and errors consistently, plus some other minor tidy-up of gs2_io. Net reduction of about 300 lines. Pulled out of #314

• Reduce duplication in gs2_io::inquire_dims and define_dims

• Reduce duplication in gs2_io::inquire_vars and define_vars

• Add message to some netcdf errors in old diagnostics

• Condense definitions of netCDF dimension arrays

• Fix gs2_io not compiling without netCDF

• Remove unused local variables in gs2_io