Fix NetCDF coordinate reads in topo_module: use varid instead of dimid

[mandli]

topo_module.f90 retrieves coordinate data using:

nf90_get_var(nc_file, x_dim_id, ...)

where x_dim_id/y_dim_id are dimension IDs and not variable IDs. The NetCDF Fortran API requires variable ID for nf90_get_var.

Because NetCDF IDs are integers, this sometimes silently works when a dimid numerically matches a varid, but fails for valid CF files (e.g., GEBCO NetCDF4) with:

NetCDF: Start+count exceeds dimension bound  (-57)

This PR simply switches to the proper variable query

nf90_inq_varid(nc_file, trim(x_dim_name), x_var_id)
nf90_inq_varid(nc_file, trim(y_dim_name), y_var_id)

and checks those coordinates.

Also adds a small fix for nearest-index searches with

minloc(abs(xlocs - xll), dim=1)

[mandli]

This now works for the GEBCO data I was testing on, but I want to change over all the NetCDF places I think this is happening while I am at it.

[mandli]

There are some changes to the util.wrap_coords function for topography that this was revealed by, but I think that will go into another PR to keep this cleaner.