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.
please use same code as for c++ code (at least for the installation, not sure if it -d make sense)

```cmake
set_target_properties( statismo-VTK PROPERTIES
  DEBUG_POSTFIX "-d"
  VERSION "${statismo_LIB_VERSION}"
  SOVERSION "${statismo_LIB_SOVERSION}"
)

install( TARGETS statismo-VTK
  EXPORT statismoTargets
  RUNTIME DESTINATION ${INSTALL_BIN_DIR}
  LIBRARY DESTINATION ${INSTALL_LIB_DIR}
  ARCHIVE DESTINATION ${INSTALL_LIB_DIR}
)
```