Event Mixing Systematic

Added a folder for systematic studies (Systematics) and a subfolder for this specific systematic (Systematics/EventMixing).

It contains two short scripts:

• select_files.py will run on the combined output of the pipeline and just selects the appropriate files for the validation comparison
• evt_mix_syst.py uses these to parameterise the difference seen in B_plus_MCORR between the validation region data and the event mixed sample. It then applies this to the nominal EvtMix template .pkl file to produce systematic alternative templates as pickled boost-histogram objects.

Note that boost-histogram > v1.0.2 should be used as a bug was spotted with older versions