Specialize on diagonal fieldvector broadcasts

[charleskawczynski]

This PR specializes on "diagonal" broadcast expressions with fieldvectors. Specifically, if a fieldvector broadcast expression (one with a FieldVectorStyle) has one and only one fieldvector type, then we don't check the broadcast axes (the combined axes is the same). The benefit of skipping this code path is that checking the axes is type-unstable (JuliaArrays/BlockArrays.jl#310). From what I can tell, there is not a simple path to fixing JuliaArrays/BlockArrays.jl#310 because:

• We want to use Tuples, so that this is stack-allocated
• Combining axes for Tuples is type unstable because the result depends on the values in the tuples.

So, I think that the simplest solution is to specialize when we know that the axes will not change.

This PR adds a test to show that is_diagonal_bc will return true/false when expected at compile time (@code_typed is_diagonal_bc(bc) returns true or false), and tests that (what was breaking) fieldvector broadcast expressions with multiple fields does not allocate.

Closes #1465.

One other benefit of this solution is that we can at some point reuse is_diagonal_bc to specialize on these broadcast expressions in copyto!, so that we perform a single cuda kernel launch (as opposed to however many fields are in the fieldvector).

The only thing remaining is that I'd like to run ClimaAtmos CI on this branch to make sure that this doesn't blow up compile-times. Update: good news, it looks fine xref: CliMA/ClimaAtmos.jl#2700 (comment)

[Sbozzolo]

I think @dennisYatunin is in a better position to review this, so I'll leave this to him.

[dennisYatunin]

Overall, I think this looks good. You could simplify is_diagonal_bc by rewriting it in terms of Utilities.UnrolledFunctions.unrolled_all, but what you have is fine. There is one change I'd like to see for catching mismatched spaces errors; see my comment.

[charleskawczynski]

@dennisYatunin, looks like we can't error: https://buildkite.com/clima/climacore-ci/builds/3150#018dc877-7cd8-4de7-928d-de25532c360d. I'm going to merge this now, and we can follow up later if there's some improvement we can make.