Move 'receiver' out of FETools::extrapolate() CellData structure

[bangerth]

Once #13212 is merged, I will do one more change: Currently, the CellData structure stores a receiver element that is an MPI rank. But this is conceptually not part of the "cell data", and it is also used after receiving data from another process to indicate the sender. A better data structure would be a map (or better multimap) mapping from the MPI rank to the CellData.

This is simply a github issue to remind myself that that should be done.