MCWeighter: allowInvalid option

By default abort if we cannot get the xsec.
But allow the user to decide.
suggest what to do before aborting
Merge branch 'master' into mcweighter-invalid-abort

Conflicts:
	Root/MCWeighter.cxx
	SusyNtuple/MCWeighter.h

Current logic now allows to (1) fail or (2) keep going, but print N
warning messages.
Merge branch 'mcweighter-invalid-abort'