Add tools for streamlining sample sizing

[dzeber]

This will integrate the tools for streamlining sample size computations prototyped in this notebook in frequentist_stats/sample_size.py.

[dzeber]

@m-d-bowerman I've been working on this as a side project on this branch. We should discuss combining with #204 , eg. maybe the pretty-printing functions go into an output class instead.

I wasn't sure how much we want to go modify exsiting functionality - I'm mainly wrapping and building on top of it.

[m-d-bowerman]

I agree on adding the pretty-printing to an output class. There's a bit of difference in output between the various sample size calculation methods, so I'm not sure how easily we can combine them into an output class.