Make P2P1TaylorHoodFunction child of BlockFunction

The merge makes the P2P1TaylorHoodFunction and the P1CahnHilliardFunction children of BlockFunction. The FunctionWrapper class is adapted, so that we can keep uvw and p as members of the P2P1TaylorHoodFunction, similar for the P1CahnHilliardFunction. Thus, we do not need to adapt the operators, solvers, ..., yet.

Note that we need to explicitely define a copy constructor in the children to take care of setting the entries in the subFuncs_ vector inherited from the BlockFunction parent. Otherwise nasty memory errors will occur.