Implement the Bretthorst periodogram algorithm for unevenly sampled data.

[barrettp]

The Bretthorst algorithm is a fully Bayesian derivation of the Schuster/Lomb periodogram or which Lomb-Scargle is a variant. In my experience the Bretthorst algorithm does a better job at suppressing aliases and therefore signals become more significant.

The algorithm is discussed in Larry Bretthorst's PhD thesis that is available as book at https://bayes.wustl.edu/glb/book.pdf .