Allow `PBC_flag` to take coordinate names

[w-k-jones]

With the shift to xarray, it would be nice to change PBC_flag from accepting hdim_1, hdim_2 to also accepting the xarray dimension names (e.g. latitude, longitude). Allowing min_h1, max_h1 etc. in coordinate variables rather than array indexes could also be added with this.

[freemansw1]

Yes, this is a good idea, although we will need to (or we should, at least) address the fact that we don't currently have an in-built hdim_1/hdim_2 to dimension/coordinate mapping. That would be good to add, but doing so would likely require changes to our output data structure (xarray would allow for this though).