Use the new Laplace assembler (in laplace.hh) to implement alpha_volume method
Implement the matrix assemble (i.e., jacobian_volume), instead of relying on an FD approximation

This is very short: we simply use the implementation in laplace.hh
Do not evaluate shape function values. They are never used.
Move computation of quadrature point factor out of inner loop

It only depends on the quadrature point and its position.  Previously
it was to deep in a loop structure, and hence computed too many times.