add workaround to read SLHA table from lhe header in memory
only load slha from lhe header in case no manual slha file was specified (allows overriding the slha header in the lhe file if desired)
Merge pull request #2170 from bendavid/py8slhaReading

Py8slha reading
Merge remote branch 'origin/CMSSW_7_1_X' into CMSSW_7_0_X