Fix crash in DTDCSByLumiSummary when MonitorElements are missing

This change properly protects the module from the case where the
MonitorElements it wants from DQMStore are not available.
This fixes a crash seen in the threaded IB RelVals.
fix bug affecting pre4 relvals
Merge pull request #4976 from VinInn/FixBug720pre4

fix bug affecting pre4 relvals
add TSG Mixing flat 20-50 mixing scenario (50 ns)
Add flat PU 20-50, 50ns
Revert "Merge pull request #4932 from inugent/7_2_X_lifetime_patch"

This reverts commit bb98411d252899f672521ac26e73f1476337a3b9, reversing
changes made to a116fda73e691b6c8d8a8ab8420605f361d973fe.
Revert "Merge pull request #4834 from inugent/TauolaLifetimePatch"

This reverts commit 03bbef518c0b786dd08e869f0cc44fb059619747, reversing
changes made to 4201a100f28ca226207fb1c3c5693f136711677f.

Conflicts:
	Validation/EventGenerator/plugins/TauValidation.cc
Merge pull request #4982 from franzoni/fw-port-TSG-pu-scenario-CMSSW_7_2_X_2014-08-18-0200

Fw port tsg pu scenario cmssw 7 2 x
Merge pull request #4986 from bendavid/revertBrokenTauFixes72x

Revert broken tau fixes (72x)
Update xml validation script.
Conventional description to retrieve xml files for validation.
Merge pull request #4989 from ianna/xml-validation

Update xml validation script
Merge pull request #4975 from Dr15Jones/fixCrashInDTDCSByLumiSummary

Fix crash in DTDCSByLumiSummary when MonitorElements are missing
Return const member from const function
Merge pull request #4997 from ianna/dd-return-const-member-from-const-function

Return const member from const function