bugfixes for PFEG: mustache conversions, GSF-SC associations, muons, mva-e-pi filling

Fix the following bugs:
- not checking to merge refineable objects through association by brem tangents (allows tracker driven gsf tracks to pull in super clusters with no associated gsf track)
- fix a bug when in GED mode where the wrong super cluster would get associated to an ECAL driven electron, this was coming from a null seed det ID in the seed PFClusters, which seems like a somewhat deeper bug.
- correctly put the mustache-driven conversions into the conversion sequence. No noticeable changes to e/gamma objects.
- correctly veto ID'd muons from becoming seeds for e/gamma objects
- Fill the electron ID mva value for gedGsfElectrons, should show up in DQM now.

The cluster/linking related fixes improve low-side (i.e. below 0.5) tails in the E_reco/E_true distribution for refined superclusters coming from electrons.

The muon fix should significantly reduce the rate of high pT muons becoming electrons.