Include DDM model

[afedynitch]

The Data-Driven model is a new parameteric hadronic interaction model based on data from fixed target experiments at accelerators (NA49 & NA61). The model draft is already available in MCEq/ddm.py.

The model has currently no tests or documentation except an example notebook that's not yet part of the source and needs to be chopped into different pieces. Better would be some simple tutorial section and an more extensive demo notebook.

So the remaining items to publish this model in MCEq are:

• Add documentation in the source
• Add tutorial section
• Add demo notebook
• Add tests