ENH: added distributions.py file and moved Beta-binomial to it

Right now we have the function `gen_probs` in career.py. This function computes the pdf (pmf) of the BetaBinomial distribution. I didn't feel like it belonged in the same file as the `CareerWorkerProblem`, as it isn't specific to that model.

This PR moves this function out of `career.py` into a new file `distributions.py`. In this file we can collect other probability distributions that aren't included in `scipy.stats` as we progress. 

I am not sure if this is a pareto improving change, so if anyone doesn't think it is -- comment here and we can find the optimal solution. 

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**Warning**: This PR might create a merge conflict with #38 when it is merged as both PRs change `career.py`.