Chain.walk(start=0) chooses a random start state

[rdbliss]

Calling Chain.walk() with the parameter start=0 chooses a random start state from the chain.
MWE:

chain = pykov.Chain({(0, 1): 1, (1, 0): 1})
chain.walk(0, 0) # Repeat a few times to get a [1].

The relevant line is here:

if not start:
    start = self.steady().choose()

This could be fixed by checking for start == None and stop == None, I think. If 0 is excluded by design, can a note be added to the documentation?

[riccardoscalco]

I think that it is bad code design and can be considered an issue. At the time I wrote pykov I was thinking mainly at states described by strings and I did not pay much attention to have a more generic codebase.

Can you please do a pull request? I'll be grateful to accept it.