CellFunction wraps dolfin::MeshFunction and not dolfin::CellFunction

[Marco Morandini]

The pybind11 wrappers of Cell/Facet/Edge/Vertex Function wrap dolfin::MeshFunction and not the corresponding dolfin derived classes. This is a problem when wrapping a user extensions that needs to receive e.g. a CellFunction, since python CellFunction maps to dolfin::MeshFunction and not to the derived class dolfin::CellFunction

[Marco Morandini]

Closing since c++ Cell/Facet/egde/Vertex Functions are deprecated.