Multigrid for non-nested levels

[peterrum]

PR #15141 introduces a two-level transfer operator between non-nested levels. There are some todos we need to tackle before merging that one and some follow-up tasks.

TODO

• Add RemotePointEvaluation::get_cell_data() -> Introduce MGTwoLevelTransferBase #15149
• Allow NonMatching::MappingInfo::reinit_cells() to work on any containers. -> Allow MappingInfo::reinit_cells() to work on containers #15151
• Introduce MGTwoLevelTransferBase -> Introduce MGTwoLevelTransferBase #15149
• Documentation.

Follow-up tasks

• multiple components -> RPE::evaluate_and_process(): pack only if needed #15163
• simplex
• other types of elements, e.g., DG
• enable interpolation to coarser levels
• hp
• enable l2 projection (potentially with intersection with CGAL)
• make parameters of RPE accessible via AdditionalData
• make RPE a shared_ptr
• RPE: precompute offsets for communication and use it during the actual communication with scaling

FYI @fdrmrc

[jh66637]

I did this for DG in a different context and can give

• other types of elements, e.g., DG

a try. I will try to do it as soon as possible :)