Include additional grouping functionality for pc and Renyi entropy

[jhenderson0]

1. Construct a pc cross function for doing general across group coincidences and work out how to average this properly
2. Integrate pc joint and pc conditional to work with pc cross. E.g. So we can compute pc across epitopes but within VA groups
3. Integrate pc cross to work with coincidence entropy so that the above options are available when working with entropy as opposed to pc.

(I already have some basic functions which do this from my 10Mil work and TCR info work - James)