WIP: Use a composite basis for the brittle fracture simulation

Conceptually the brittle fracture simulation uses two spaces: one for the deformation and one for the damage fields. Previously, these were squeezed into a single power basis with dim+1 coefficients. This works, but it sometimes requires manual index manipulations, which are not always obvious. This patch introduces a proper composite basis that separates the deformation from the damage.

The multi-indices for the global degrees of freedom remain unchanged!

WIP because it contains the idea, but doesn't even compile yet...