Added a 2D histogram to monitor MC vertices vs BX number: 2nd step

Conflicts:
	Validation/RecoVertex/src/MCVerticesAnalyzer.cc
	Validation/RecoVertex/test/mcverticessimpleanalyzer_cfg.py

Conflicts:
	Validation/RecoVertex/src/MCVerticesAnalyzer.cc
	Validation/RecoVertex/test/mcverticessimpleanalyzer_cfg.py