fix beginLuminosityBlock, and add endLuminosityBlock and endRun

This pull request fixes two problems in the DataMixingModule:
1) DataMixingModule::beginLuminosityBlock() did not call BMixingModule::beginLuminosityBlock(),  This causes a segfault if the secondary source fails to open an input file.
2) beginRun and beginLuminosityBlock were implemented, but endRun and endLuminosityBlock were not.  This is an accident waiting to happen.  The implementation of endRun and endLuminosityBlock do not change current functionality, but make the code more maintainable.