New calculate_observable function and moved the relative imports

New 'calculate_observable' function in webapp_backend to obtain the
eventrate or differential cross section as expressions instead of the
correlation matrices. Furthermore I have played with the relative
imports (the ugly sys.path.append stuff) and tried to move them to the
__init__.py files.
Moved the main execution file (mgeosm2observables.py) up for easier
introduction.