Adding Bayesian SA + corresponding test

[larryshamalama]

Closes #5

[larryshamalama]

I think what we can do is as follows: we can test if the Bayesian analysis is correct from a purely code perspective by including $U$ in the outcome model and see if our gamma and beta parameters converge (across a few simulations with appropriate burn_in and n_samples, maybe 10). This is already completed in this PR

Because of non-identifiability, it would be then important to investigate situations where we can better identify the treatment effect will converge to its true value. This can either be done using a restrictive data-generating procedure or, hopefully, using informative priors. This can be done in another PR