MCWeighter: add dump* methods

- make dumpSumwMap const
- implement dumpXsecCache, dumpXsecDb
test_mcWeighter: test one specific sample with input tree
MCWeighter: warn only for MC, with proc info
add MCWeighter::ProcessValidator class

This class is used to keep track of the events where the process id is
not identified correctly. This can happen (not very often) when
creating SusyNtuples for SUSY samples. We decided to skip these events
(by setting the xsec to 0.0) and print a summary at the end.
Merge pull request #25 from gerbaudo/dbg-mcw-susy

Dbg mcw susy