Regridding - for land sea mask

### What problem does your feature request solve?

At the moment, and by design, the land sea mask only works for data with the same shape as the mask (i.e. not staggered variables).

### Describe the solution you'd like

This may link into wider re-gridding considerations. Otherwise a simple regrid step would be needed.

### Describe alternatives you've considered

Start by locating the land-sea mask implementation and any existing regridding code; the issue names no files or tests. Clarify how staggered variables should be handled and whether this belongs in broader regridding work. Done should allow the land-sea mask to operate on data with a different, staggered shape.

Written by the indexing model from the issue text.