Changes to FIAT to enable FUSE

[indiamai]

Minimal changes to enable FUSE in FIAT.

Key changes:

• Avoiding specific references to UFC cells
• Lists of subentities can optionally be passed to reference cells
• FUSE infrastructure for element creation at FIAT, finat and UFL level. Most notably the inclusion of use_fuse and FuseElement. use_fuse is derived from the Firedrake mesh.
• Moves as_cell into FIAT from UFL so that we can delegate to fuse creation as appropriate. Falls back to UFL implementation in all other cases.

Tested in firedrake on Firedrake PR #5276 and FUSE main sits on this branch.

[JHopeCollins]

Could use a string enum here: https://docs.python.org/3/library/enum.html#enum.StrEnum

Suggested change

	class CellBackend(Enum):
	FIAT = 1
	FUSE = 2
	class CellBackend(enum.StrEnum):
	FIAT = 'FIAT'
	FUSE = 'FUSE'