Revise mechanism for setting bonus parameters.

* Add methods for computing relative distance between
  parameter sets over actual model params to model classes.
  paramRelativeDistance.
* Add setBonusParams to calcLike/calcLikeDouble.
  Now will compute the bonus params if they aren't set
  or the new param set is too far away from the one used to
  compute the previous value (based on the above relative
  distance functions), but tries to re-use the values
  from getLogLike if available.
* Set the mean flux internals to NaN before checking validity
  to make sure invalid values don't leak through from previous
  call.