Backtracks

Thanks to tomasstolker , `backtrack` can now be used on clusters using `schwimmbad.MPIPool`

new also is the `set_prior_attr` function and new options in the `fit` function, like nlive=500, mpi_pool=False, resume=False.

More to come (maybe someday I'll write docstrings)!

Hello world!

`backtrack` is here, and I'm version-ing the code here so we can continue development. Lots of work yet to be done (like docstrings, lol) but we can reproduce the results of [Nielsen et al. 2017](https://arxiv.org/abs/1705.06851), and the code appears applicable to at least one other case (citation TBD), which was the intention of the code in the first place.