Partitioning of lbc meshes for lfric2lfric

### Are there any linked Issues or Pull Requests?

MetOffice/lfric_apps#343

### Brief description

Partitioning of lbc meshes is necessary for lfric2lfric to generate lbc data in the lbc mesh for performance reasons. While for running lfric it is probably necessary to match the partitioning of the regional and its lbc mesh, in lfric2lfric this limitation is not present, so a dedicated partitioning will be written for performance reasons.

### Further details of the issue.

_No response_

Start with the linked issue MetOffice/lfric_apps#343 and locate the lfric2lfric mesh-generation entry point and existing mesh-partitioning implementation. Determine the required dedicated partitioning behavior for lbc meshes and define completion around generating correctly partitioned lbc data with the intended performance benefit.

Written by the indexing model from the issue text.