new SNRPDF mechanism:

- introduce internal cacheentry class to hold entry information
- remove limit on cache size
- when initializing, move loop over instruments into .add_to_cache()
- add min_instruments parameter to the cache key
- in P_instruments_given_signal(), handle off instruments better
- record P(instruments | signal) in the SNRPDF cache
- enables generalization of ranking statistic away from fixed P(instruments | signal) marginalized over horizon distance history