slate: fix kernel argument ordering

[ksagiyam]

Fix #4001.

Consider a slate expression:

T = A + B

where:

A = 0 * f + g
B =  f + g

Without aggressive zero simplification, we had:

A.coefficients() = (f, g)
B.coefficients() = (f, g)

which lead to T.coefficients() = (f, g, f, g)->(f, g) == B.coefficients().

Current implementation in Slate had happened to be working as B.coefficients() had been the same as T.coefficients().

With aggressive zero simplification (now merged in UFL), we have:

A.coefficients() = (g,)
B.coefficients() = (f, g)

which lead to T.coefficients() = (g, f, g)->(g, f) != B.coefficients(), and the current Slate breaks.

This PR is to fix this issue.

[connorjward]

Makes sense. Thanks for taking this on.

[github-actions]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake real	8083 ran	7367 passed	716 skipped	0 failed

[github-actions]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake complex	8111 ran	6561 passed	1550 skipped	0 failed

[pbrubeck]

Well done @ksagiyam! This is probably the Coefficient analogue to what had to be done when Constants are present in a mixed problem but not on every subblock: 6078f93

[pbrubeck]

Looks good to me.

[JHopeCollins]

If we leave this test here for now (and I do think we should until downstream testing gets a more formalised solution), then at least we can make it a bit smaller.
a) the solve doesn't need to be accurate, it just needs to run.
b) the second solve to reconstruct buoyancy doesn't test anything that isn't definitely covered elsewhere (it's a fairly simple form).

I'm happy to merge with these updates.

[JHopeCollins]

Approved pending CI passing.
@connorjward Do you know what this error in the checkout action means?

Error: File was unable to be removed Error: EACCES: permission denied, rmdir '/__w/firedrake/firedrake/firedrake/.github/ISSUE_TEMPLATE'

Can I just try rerunning it or does something need fixing?

[connorjward]

Approved pending CI passing. @connorjward Do you know what this error in the checkout action means?

Error: File was unable to be removed Error: EACCES: permission denied, rmdir '/__w/firedrake/firedrake/firedrake/.github/ISSUE_TEMPLATE'

Can I just try rerunning it or does something need fixing?

I think this is a side effect of me messing about with CI for #4011. I believe I have fixed this and that it shouldn't recur. If you notice this again please let me know.

[JHopeCollins]

Ok, I've set the failed job off to rerun.