Make ensembles into subclasses of IRSystemBase #11
Description
Since #6, we've had ensembles available in the pv211_utils.ensembles module as functions. It seems more useful to treat ensembles as systems, i.e. let's move ensembles.py to system/ensembles.py and make each function into a subclass of IRSystemBase? See also a related comment from the review of #8.