Major refactoring for parallel agglomerated multigrid

[fdrmrc]

This PR hopefully provides a stable interface for agglomerated multigrid. Changes propagated almost everywhere, but meaningful new examples have been added:

• time dependent monodomain problem, where agglomerated multigrid is used as a preconditioner
• combination of matrix-free operator evaluation inside agglomerated multigrid

plus several utilities in utils.h and poly_utils.h that allow easy integration of the present infrastructure in existing codes.

The huge number of lines is due to the addition of two 3D ventricle meshes.

[fdrmrc]

LinearOperator does not provide m() and n() and el() members, which are needed by the PreconditionChebychev class. As a temporary solution (before opening a PR upstream), I defined a new class LinearOperatorMG by copy-pasting the LinearOperator headers and adding the necessary members. Definitions are in /include/linear_operator_for_mg.h