[pool process + processing thread] proto - make processing in a pool and start from a separated thread to avoid gui freeze.

Move Normalization to multiprocessing.Pool and the call from orange from a dedicated thread and with a QThreadPool.
This prevent any gui freeze.
For now this is just a prototype. Some thing to do:

- create a pool_process that will be inherited from PyMca_normalization and QPyMca_normalization.
- generalize this structure for each orange widget.