Numpy like utility for versioning

[adler-j]

Currently we have a version number of ODL in a few places (I know at least 2, setup.py and init.py).

Numpy has a utility which creates a version.py file on installation, hence enforcing one and only one version number throughout the package. I think this is something we could use.

[kohr-h]

Would be good, agreed. I once looked into their solution, too, but didn't bring enough time to understand it properly.