Adapt Dune::Fufem::DuneFunctionsOperatorAssembler for the clever use of a ScaledIdentityMatrix and Powerbases

When assembling a mass matrix using a Powerbasis of dimension N as the ansatz and trial, one cannot work with 1x1 block matrices but needs to use NxN block matrices. The mass matrix then contains NxN block matrices containing the same value on the N diagonal entries. A better solution would be to use a ScaledIdentityMatrix for this.