Disable IMT for merging of NanoAOD.

As documented in CMSCOMPPR-3348, IMT triggers a crash in ROOT due
to the fact that NanoAOD adds branches to the tree.  This patch
is a simple workaround - we disable IMT for the merge jobs (which
are single core anyway) - until the ROOT issue is resolved.

Configuration/DataProcessing/python/Merge.py

@@ -66,6 +66,7 @@ def mergeProcess(*inputFiles, **options):
     elif mergeNANO:
         import Configuration.EventContent.EventContent_cff
         outMod = OutputModule("NanoAODOutputModule",Configuration.EventContent.EventContent_cff.NANOAODEventContent.clone())
+        process.add_(Service("InitRootHandlers", EnableIMT = CfgTypes.untracked.bool(False)))
     else:
         outMod = OutputModule("PoolOutputModule")