refactor interpolation

[leo-collins]

An attempt to clean up interpolation code. Currently includes #4582. So far:

• Removed interp_data dictionary in favour of the InterpolateOptions dataclass. We get type hinting, better IDE support, single source of truth for these options.
• Interpolator becomes an internal object. I removed the __new__ method and dispatch instead with a get_interpolator function.
• We can now matfree adjoint interpolate cross-mesh and VomOntoVom. The renumbering malarkey is completely removed.
• Can now assemble cross-mesh interpolation operators, both forward and adjoint.