Implement parallel transfer interface between agglomerated multigrid levels

[fdrmrc]

An implementation of a parallel transfer operator between possibly agglomerated levels is provided, named MGTransferAgglomeration. This class just takes:

• the intergrid transfers
• DoFHandlers for each level

and implements the interface required by MGTransferBase, providing prolongation and restriction methods across multigrid levels. Together with #109, it allows the usage of such transfer operator in a multigrid preconditioner.

The fact that the triangulation is always the same on each level implies that the copy_to/from levels can simply be performed by copying locally owned data. Tested locally on some unstructured meshes and with polynomial degrees up to $3$

Builds on top of #109, only relevant commit is 5b7f091.

[fdrmrc]

Compilation fails since the LinearOperator class does not provide m() and n() and el() members. I've added them to my local deal.II build.