Further Heppy bugfixes

Some bugfixes that didn't show up until things were cleaned up also from CMGTools side and more testing was done:
   * `heppy` did not work multithread when using the `AutoFillTreeProducer` since the config is not pickleable (re-importing didn't work, as the cfg can modify sequences or objects, and when running with `multiprocessing`  the modification was being done twice)
   * also, get stack traces when running within `multiprocess'
   * `from ROOT.heppy import TriggerBitChecker` doesn't seem to work anymore after cleaning up `CMGTools.RootTools` from my working area (no idea why) but `ROOT.heppy.TriggerBitChecker` works
   * `GeneratorAnalyzer` was by mistake also in `objects.all` and not only in `gen.all`

@arizzi @mariadalfonso