pythonpath_reduction: Corrections

pythonpath_reduction: Corrections

- Minor correction. Actually, we don't need the prerequisites install dir

- Upgrade pythonpath_reduction.py script
  Using argparse, add options to customize :
  * salome-install-dir
  * context-file name
  * env-file name

- Backup of files is done. These backups are used at start if they exist.
  This allows to use this script several times.

  On linux, in the python_modules directory, symlinks are created instead of copies.

- Fix remove link and add ignore option

- Only one directory is mandatory

- Install site-patch.py from setuptools

- Minor correction; get absolute path of salome_install_dir, embed paraview pythonpath into python_modules