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!
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