store factorized logprob

Fixes #18 (closed).

Note! the names assigned to the factorized logprob values are computed at the time the events are generated (if EventGenerator is told to cache these numbers). Those names do not automatically update when we change the naming convention. This was deemed acceptable.

The main roadblock to supporting dynamic/automatic renaming of factorized logprob is setting up the equivalen of backends.names.invname for the mangled logprob names. This may be possible in the future, but was not thought to be worth the development headache until a need for this functionality was demonstrated.