Add Tree.hs: binary trees with two types of label
Use Tree instead of Map to implement Classed.
Delete unused map-of-lists code from analyze.hs
Use Tree to implement Model directly, not via Classed.
Delete unused Classed datatype from analyze.hs
Compute weights efficiently when computing them all at once.
Delete superfluous import of Control.Applicative
Factor out common parts of sampleLogLikelihood and weights
Report sizes of classes in progress report from analyze.hs
Refine models by splitting individual leaves of classification tree.

(As opposed to splitting all leaves simultaneously, which generates
a Cartesian product.)
Name the data type of leaf values in models.
Allow features to be used in multiple branches of a classification tree.