In detail we can now call the following free functions using objects
of type P[1,2]VectorFunction
- createVectorFromFunction()
- createFunctionFromVector()
- applyDirichletBC()

Commit also adds a simple VectorFunctionPetscTest that checks that
conversion to and from works.

We also rename the first argument of createFunctionFromVector() in
PETScVector from src to dst, as this seems more self-explanatory.