Allow for solving multi-domain problems involving codim-0 submeshes #1600

Job	Run time
test / Build and test Firedrake (Linux) (default)	1h 4m 21s
test / Build and test Firedrake (Linux) (complex)	43m 2s
test / Lint codebase	28s
test / Build documentation	10m 11s
test / Build and test Firedrake (macOS)	0s
test / Deploy GitHub pages (optional)	0s
test / Upload to PyPI (optional)	0s
	1h 58m 2s

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