Split products.hpp and product_mixture.hpp out of product_model.hpp
Optimize read_value; add ValueSchema::validate(features)
Move towards sparse-cat branch
Add explicit ProductMixture::maintaining_cache variable
Allow mixture validation when not maintaining_cache
Move mixture_init_unobserved to cross_cat.cc
Merge pull request #36 from priorknowledge/maintaining-cache

Be explicit about when mixture cache is maintained
Merge remote-tracking branch 'origin/master' into preql
fix typos