SNSPowderReduction is broken when PreserveEvents=False

[martyngigg]

It was broken in this [https://github.com/mantidproject/mantid/commit/342aca3e08d3ec21be2627f2d380f85b8bb72bfb#diff-1b114ee94e8df86b22e2cac0469acd8b commit] with the addition of a logging statement that asks for the number of events (342aca3#diff-1b114ee94e8df86b22e2cac0469acd8bR660)

This has caused the [http://builds.mantidproject.org/view/Nightly%20Builds%20and%20Tests/job/master_clean_rhel6_mpi_systemtested/lastCompletedBuild/testReport/SystemTests/SNSPowderRedux/SeriesAndConjoinFilesTest/ MPI system tests] to fail as the PreserveEvents flag is force to false for an MPI build.

This script demonstrates the problem (based on the current master)

#!python

cal_file   = "PG3_FERNS_d4832_2011_08_24.cal"
char_file  = "PG3_characterization_2012_02_23-HR-ILL.txt"

# reduce a sum of runs - and drop it
SNSPowderReduction(Instrument="PG3", RunNumber=[9829,9830], Extension="_event.nxs",
           Sum=True, # This is the difference with the next call
           PreserveEvents=False, VanadiumNumber=-1,
           CalibrationFile=cal_file,
           CharacterizationRunsFile=char_file,
           LowResRef=15000, RemovePromptPulseWidth=50,
           Binning=-0.0004, BinInDspace=True, FilterBadPulses=True,
           SaveAs="gsas", OutputDirectory=os.path.expanduser("~/"),
           FinalDataUnits="dSpacing")

[martyngigg]

This issue was originally trac ticket 10225