[BioPython] Markov Model module in BioPython

[Pepe Barbe]

Hello Everyone,

I am curious if the Markov Model module works, of it is complete. I've
seen the tests for the module and it doesn't test the functionality of
the entire module and I've ran into some issues that make me wonder if
it is complete.

E.g. the allow_transitions function in the Markov Builder class is
missing self._state_alphabet.letters in line 182. After fixing that,
another bug showed up. Thus I wonder if it has been tested or if its
functionality is complete.

I am running BioPython 1.42.

Thanks,
Pepe