Add argument to enable systematics regex from CLI
This adds the ability to specify a set of regexes for systematics from the command line argument with:
easyjet-ntupler -y True -x [regex_1] [regex_2] ...The function defined in systematics_helper.py then merges these into a full regex for the SystematicsSvc, of the form:
(^$)|(^regex_1.*)|(^regex_2.*)|...Not specifying any -x argument instead simply uses .*.
The list can also be specified in the run config, for longer and more specific sets that may not reduce neatly to a small number of regexes.