XIOS: Mask should be treated separately

[jwallwork23]

Once #934 and #910 have been merged, we should account for special treatment of the landmask (and possibly other fields?).

The landmask is fixed and so should only be written once. This can be achieved using the XIOS 'once' field operation setting.

Moreover, it should be a diagnostic output rather than a restart.

[jwallwork23]

Beginnings of work on this on branch https://github.com/nextsimhub/nextsimdg/tree/issue935_xios-landmask