MatrixFree: Templatify reinit functions

MatrixFree: Templatify reinit functions

Unfortunately, we have a bit of a clashing ideology here:

 - The MatrixFree container is motivated by the old ConstraintMatrix
   object. Meaning it expects constraints to be expressed in double.

 - AffineConstraints<number>, however, should be matched with the
   underlying number type. And the rest of the library now assumes that
   this is the case (in particular project_matrix_free and variants...)

In order to facilitate both approaches, we have to slab another "number"
template aorund the reinit functions. That's a bit unfortunate but
cannot be avoided. :-/