Model 3

This tags a stable(ish) version of Model 3 as described in the "GenDB - HIRM with PClean Entity Resolution" doc.

The schema is the same as the original HIRM, except that additional distributions and data types are supported (a bigram distribution over strings, a normal distribution over floats, categorical distributions over ints/strings, and a few others, in addition to the Bernoulli distribution in the original HIRM code). Most of these are specified in the schema simply as the name of the distribution (e.g. "normal") but a couple are more complicated -- see util_distribution_variant_test.cc for examples.