lfric2lfric: lbcs written in a lbc mesh

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

MetOffice/lfric_core#301

### Brief description

lfric2lfric is able to generate boundary conditions for running regional models. At the moment, these boundaries are written in the full regional mesh, containing zeroes in the middle of the domain. These means that the boundary files are quite large, which is a great obstacle in the development of high resolution regional LFRic configurations. For efficiency, it would be desirable to write the boundaries in lbc meshes, as they will contain information just on the lbc rim.

The problem at the moment is the partitioning of the lbc mesh, so some code will need to be created for this purpose. 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 necessary, so a dedicated partitioning will be written for performance reasons.

In lfric2lfric we will need to read the source mesh, the destination mesh (due to the need for reading an orography file), and the lbc mesh, so we will need three instead of two XIOS contexts.

### Further details of the issue.

LFRic is at the moment not capable or reading or writing lbc meshes, so extra code will need to be added to lfric_core to make these lbcs in the lbc mesh usable by LFRic.

Start with the lfric2lfric implementation and the linked MetOffice/lfric_core#301 issue. Trace how source and destination meshes, orography, and XIOS contexts are currently handled, then investigate the required lbc-mesh partitioning and LFRic support. Done means boundaries are written to lbc meshes and can be read and used by LFRic.

Written by the indexing model from the issue text.