Provide trainers which estimate parameters based on training sequences.
These should be used to train a Markov Model prior to actually using
it to decode state paths. When supplied training sequences and a model
to work from, these classes will estimate paramters of the model.
This aims to estimate two parameters:
a_{kl} -- the number of times there is a transition from k to l in the
training data.
e_{k}(b) -- the number of emissions of the state b from the letter k
in the training data.