Revision of numdom handling.
Default (=0) use DOMAIN_number_total global attribute.
Command line argument (-n) now takes precedence, so that we can overwrite the global attribute.
This is useful when the output use XIOS's multiple_file mode in simulations where NEMO use a total number of MPI tasks greater than the number of latitudes. In this scenario the number of subdomains is equal to the number of latitudes due to limitations in XIOS, but the DOMAIN_number_total global attribute is equal to the total number of MPI tasks used by NEMO at runtime.