feature request: memory models

it would be really cool to integrate some models into Quail at some point (e.g. second release)

for example:
- rather than building a separately implemented mechanism into AdaptiveFR, we could call Quail functions to get the order of the next list
- our current experiment uses clustering scores to order future lists
- we could instead use arbitrary functions of prior data (e.g. models fit to prior data) to order those future lists

in addition, it'd be nice to have an easy way for people to fit some popular memory models to their data. (we could start with CMR or pTCM.) i'd love to also include marc howard's multi-timescale version of TCM, but that one is tricky to implement, so perhaps not a good place to start.