Py3: make 'Toolbox/FR.py' compatible with Python 3.3
Py3: make 'Toolbox/DSSRT_tools.py' compatible with Python 3.3
Py3: make 'Toolbox/PRCtools.py' compatible with Python 3.3
Py3: make 'Toolbox/ModelHelper.py' compatible with Python 3.3