Ideas for accelerating PTM computation
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First thing to note is that codebook computation now takes up the
majority of the time spent evaluating PTMs.  So speeding up Gaussian
evaluation is suddenly important again.

Using a tighter top-N beam will speed up Gaussian computation by
imposing a higher floor on densities, but this effect isn't worth a
whole lot, in contrast to SC models where mixture computation rather
than density computation is the most expensive part.

This means that we should probably bring back kd-trees, although the
implementation should be tweaked to be faster loading.

Also, maybe more importantly, we can do some form of CI-GMM selection
on the codebooks.  This won't actually work with the way the models
are set up currently since the CI phones share the same codebook as
the CD ones, and the goal is to prune codebooks rather than phones.

But wait!  It's okay, because we still have the same top-N mechanism
as before.  We can use those top-N scores to do early pruning of
entire codebooks.  This ought to give us the most bang for the buck.
