Revisit multigrid operator construction and full-assembly

[sebastiangrimberg]

NOTE: Based on #166

Summary of changes:

• Coarse operators for p-multigrid reuse the quadrature data from the fine level to reduce the need for JIT compiling a new QFunction and assembling new quadrature data. This is the approach from https://arxiv.org/pdf/2204.01722.pdf.
• Refactor coarse operator parallel full assembly to be in one place. This should enable Complex-valued coarse solver #142 by some relatively simple changes inside of MfemWrapperSolver.

[hughcars]

LGTM. I tried to think of a simpler control flow for the PtAPr/i and hr/i section but most just end up in more of a mess so I think the mild complexity there is totally valid.

[hughcars]

debris?

[sebastiangrimberg]

I'd like to leave these in (as with #167 (comment)) in case eventually we do want to activate this (though for now it does appear to not be worth the extra memory overhead).