Our usual tag-based approach to the numberOfLocalDoFs() free functions
cannot work with Generic- and BlockFunctions, as for those we need an
object instance of the class to determine that value.

In order to remain somewhat compatible with the current approach this
commit adds a version of numberOfLocalDoFs() that takes as input a
function object. For our standard function classes work is then delegated
to the existing free functions by determining the function tag. For the
Generic- and BlockFunction we have special implementations that call
their getNumberOfLocalDoFs() member function.

For the case of the GenericFunction this member function is implemented
by this commit. As usual it is purely virtual and the actual implementation
happens in the FunctionWrapper class.

We use this new function variant in the construction of PETScVector for
testing and as this is where we will need it when converting
GenericFunction objects to PETScVector objects.

Additionally this commit implements enumerate() without an offset in
GenericFunction and FunctionWrapper.