Store cross section into the output

#87 added a printout of the cross section as calculated by Pythia8 to the standalone executables that use Pythia8. It would be nice if this information would also be stored in the created output file somehow.

I suppose this mainly concerns the standalone case as for the framework integrated usage this is probably more in the realm of k4Gen.

Start by reviewing the cross-section printout added in issue #87 and the standalone executables that use Pythia8. Trace how those executables create their output file, then determine how the cross section should be stored there; done means the value is present in standalone output files without changing the framework-integrated usage.

Written by the indexing model from the issue text.