Move development of generators to separate package

[gipert]

Since primary vertex sampling does typically not require tracking (G4) but mostly geometry (not part of remage), we should do that with independent software and read in the output in remage.

We need to discuss:

• which new packages do we need (how many, names), which programming language
• standard format accepted by remage
• efficient parsing of vertices by remage

Related: #156 #56 #45

[tdixon97]

• What actually needs to be done here? I think we made most of the decisions (see Expand vertex file reader to parse particle type and momenta #156 ). So the issue is now just the reading which @ManuelHu is working on (done?) and then the python generators code?