NG-ARCH: Loop blocking for vertical layers

[sebvdb]

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.

Further details of the issue.

Further details in #978.

This part of the issue covers blocking of apply_mixed_operator_kernel.