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.
PyUnit (which is used for running the python unit tests) does not have a mechanism for passing command line arguments. Therefore the data directory on which the tests run is hard-coded. As the relation between source and target directory is not known, the easiest solution was to copy everything to the build tree.  On the other hand, I anyway gave  up to try to run the unit tests directly using cmake now (setting the Pythonpath correctly is too difficult). If you prefer we can also just keep scripts and data in the source tree.