Remove all references to recalc_imodel( ) and cached_alignment_prior.

We no longer need to watch for changes to Heat.beta or IModels.training
in Parameters::recalc( ), either.
Cache sequence lengths using the new framework.
Force parameters to be created before we copy the model.

This causes problems if they are created later with a different token.
Remove note_sequence_length_changed( ) functions, both variants.