Accept totalUnits for LumiBased and FileBased.

Allow the user to specify Data.totalUnits for LumiBased and FileBased splitting.
The patch was tested using the /GenericTTbar/HC-CMSSW_5_3_1_START53_V5-v1/GEN-SIM-RECO input dataset, which contains 177 files.
When I specified Data.splitting = 'LumiBased', Data.unitsPerJob = 10, Data.totalUnits = 5, http://glidemon.web.cern.ch/glidemon/jobs.php?taskid=383730, one job was created with 5 lumis. These 5 lumis were 671695, 671721, 671746, 671840, 672623, which are not the first 5 lumis in the dataset.
When I specified Data.splitting = 'LumiBased', Data.unitsPerJob = 10, Data.totalUnits = 10, http://glidemon.web.cern.ch/glidemon/jobs.php?taskid=383729, one job was created with 10 lumis. These 10 lumis were 670969, 671693, 671721, 671789, 671802, 671840, 671867, 671882, 671884, 672610, which are not a superset of the previous 5 lumis.
I ran a third task putting again Data.splitting = 'LumiBased', Data.unitsPerJob = 10, Data.totalUnits = 5, http://glidemon.web.cern.ch/glidemon/jobs.php?taskid=383765, and the 5 lumis were again 671695, 671721, 671746, 671840, 672623.

This is for issue #4367.