fixes to pythia8 pt gun

made anti-particles back-to-back to particles.

made quark + anti-quark and gluon pairs colourless states.
code stolen from pythia8.185 examples/main21.cc

tests:
- generated 10 events for each id [1,5,6,21,22,23]
- checked logs for pythia errors
- visual inspection of events with cmsShow

test outcome:
- all ok