Modernize xgen test

Use ThOr functors as much as possible in example. Add a description so users can use the example to test decay files. The goal is to advertise this at https://gitlab.cern.ch/lhcb-datapkg/Gen/DecFiles/blob/master/CONTRIBUTING.md .

More functors can be added as required by the simulation project. Please comment here.

Related to but not (yet) dependent on Analysis!846 (merged)