NG-ARCH: Loop blocking for vertical layers

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

Original trac ticket: ticket:978
Linked core issue: MetOffice/lfric_core#219
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 [#978](https://code.metoffice.gov.uk/trac/lfric_apps/ticket/978).

### Further details of the issue.

Further details in [#978](https://code.metoffice.gov.uk/trac/lfric_apps/ticket/978).

This part of the issue covers blocking of `apply_mixed_operator_kernel`.