Change lnd_get_global_dims to use ncd_inqfdims or simplify this routine to assume a certain file format

From a quick look, it looks like most of the new lnd_get_global_dims subroutine (everything after the call to ncd_pio_openfile) essentially duplicates the logic in ncd_inqfdims (in ncdio_pio.F90.in). It would be good to reuse that subroutine instead of duplicating the logic here.

This is emerging from https://github.com/ESCOMP/CTSM/pull/1258, but we are holding off on making this change because it will require some extra testing. Once we have more testing in place for nuopc in the aux_clm test suite, we should consider making this change.