UD-Graph refacto

[DiegoVinasco]

UD-Graph currently implements most of its code as python scripts (e.g. using if __name__ == "__main__": do stuff). This means that these scripts cannot be properly imported into one another. The current work around is to use os.system to call other scripts through the command line but this is quite hacky and not OS agnostic.

It would be better to implement UD-Graph python code as a python package(s) in order to allow these scripts to be properly imported into one another (and eventually be reused by other libraries projects)