NG-ARCH: Loop blocking for vertical layers

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

Original trac ticket: ticket:4680
Linked apps issue: MetOffice/lfric_apps#136
PR:

### Brief description

In order to aid vectorisation, the vertical loops over k can be blocked with a tunable compile-time BLOCK_SIZE parameter. This is the the GitHub version of a previous Trac ticket [#4680](https://code.metoffice.gov.uk/trac/lfric/ticket/4680).

### Further details of the issue.

Further details in [#4680](https://code.metoffice.gov.uk/trac/lfric/ticket/4680).

This part of the issue covers blocking of `matrix_vector_kernel` and the definition of the `BLOCK_SIZE` parameter in `constants_mod`.

Start by reading the linked Trac ticket #4680 and the linked lfric_apps issue #136 for the missing implementation details. Then locate matrix_vector_kernel and constants_mod in the LFRic core. Done means vertical k loops in matrix_vector_kernel support blocking controlled by a compile-time BLOCK_SIZE parameter defined in constants_mod.

Written by the indexing model from the issue text.