Python Wrapping for VTK adapted to new module structure.

Added option to build Python Wrapping. It works and all (Python) unit tests run successfully

* It is only tested on linux
* It requires that VTK is compiled using Python support (which is not turned on in the superbuild)
* The PYTHONPATH has to be set manually before the tests can be run

As we should anyway move to a new wrapping (using WrapITK) in the future, we can probably live with these restrictions.
@marcelluethi not sure from where _statismo comes from, here? not sure also about this dependency (since HDF5 is not a target of the project anymore)?