#3315 Loop fusion of builtins on the same any_space_

[mo-alistairp]

When multiple builtins in an algorithm are on the same any_space_ they should iterate over the same number of DoFs. Hence, these are candidates for fusion. This is currently missed in the LFRic loop fusion validation step and can be extended to allow this increased functionality. This change should allow the removal of some PSyKAl-lite code in LFRic.

[mo-alistairp]

To note, I've changed test_loop_fuse_error to instead check for correct fusion. This does simultaneously cause a reduction in test coverage in cases where there are builtins on any_space_ but these are different spaces (i.e., the TransformationError on lines 188-192). I'll need to add that coverage too