Generator Interface backports for Pythia 8, LHE input, jet matching

Pythia 8 interface and LHE interface/input have been brought fully up to date with 7x series.  Jet matching is targeting pythia 8 internal matching/merging.  "Legacy" matching interface has not been touched.  (So Pythia 6 and fortran ME2pythia behaviour should be completely unchanged)

Needs to go in together with update to Pythia 8.200pre2
https://github.com/cms-sw/cmsdist/pull/1063

In order to be strictly a backport, the last pull request should be merged into 73x first in principle:
https://github.com/cms-sw/cmssw/pull/5590