TODO

v1.0 : stuff to add
  abstract out end-node and response models
    implement at least one other model type
  categorical variable detection/use
    cleanup old array of bools vs. bits in a uint
    check that using values of uints hiding w/in doubles makes sense
  warn on possible factors
  change and swap rule variable names
  change and swap rule encapsulation
    unify node state store/restore
  "functions" getting a better organizing principle
  general encapsulation of node implementation
  look into speed change in tracking variables avail for split
  move n.eff cached calculation into scratch
  handle updating y for binary better w/r/t latents

v1.1 : cleanup, bug fixes
  thread cancellation
  check bounds on parameters for CGM prior
  verify that all of state is necessary to store