COREG:
- some modifications in order to ensure Python 2.7 compatibility
- pyfftw is now only imported if available

Geom_Quality_Grid:
- some modifications in order to ensure Python 2.7 compatibility

MAIN:
- CoReg_Sat root directory is automatically added to PATH
- py_tools_ds root directory is automatically added to PATH (expected to be in CoReg_Sat root directory)