Unify and improve tauchen and rouwenhorst

[jstac]

The arguments to these two functions have different order, even though they do something very similar.

I think it would be worth breaking backwards compatibility to fix this. We just need to take care to update the lectures accordingly.

Also, ybar is missing from tauchen and it would be good to have it.

See http://quanteconpy.readthedocs.io/en/latest/markov/approximation.html

I propose that we change the signature of tauchen to tauchen(n, ybar, sigma, rho, m=3).