u.sub(0) should still be in a ComponentFunctionSpace if the vector is 1 dimensional

[JHopeCollins]

Currrently, for u in a VectorFunctionSpace, the Functions returned by u.sub are in different types of spaces depending on the dimension of the VFS.

• if dim is 2 or higher, then u.sub(i).function_space() is a ComponentProxyFunctionSpace.
• if dim is 1, then u.sub(0) is u, instead of being in the ComponentProxyFunctionSpace.

This PR changes the u.sub(0) for 1-dim vectors to be in the ComponentProxyFunctionSpace by changing the if-statement to test self.rank instead of self.dof_dset.cdim.
Change made here:

firedrake/firedrake/function.py

Line 127 in a2a9448

if self.function_space().rank == 0:

The rest of the changes are making how/where the mixed/vector-ness is tested in subfunctions/sub/_components consistent across functions and function spaces.

[github-actions]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake complex	8125 ran	6540 passed	1585 skipped	0 failed

[github-actions]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake real	8131 ran	7345 passed	786 skipped	0 failed

[connorjward]

Basically just f-strings