Add a processor for low-mass Higgs

Although there are similarities between SM Higgs measurement and low-mass Higgs search. Some selections are hardcoded in the BaseProcessor, e.g. photon pt thresholds, triggers, electron veto, etc. The low-mass Higgs search uses different selections than the SM measurement. To avoid manually editing these selections every time after updating HiggsDNA, it is better to have a dedicated low-mass processor.