Fix multiple MC file lists

Release notes

Fix implementation of multiple MC file lists (!637 (merged))

Details

It turned out that I didn't properly test my implementation of !637 (merged) (the output of initialize.py looked good and, on a quick glance, it seemed to work). TQSampleInitializer::readInputFilesList does discard all files it already has stored, thus only the samples listed in the last file of the input file list were properly processed. This MR adds an option to skip the call of TQSampleInitializer::reset and uses this to really have multiple MC file lists. For data, it already worked correctly in !637 (merged).