Path sampling model comparison tutorial in BEAST2

[alexeid]

Model comparison is a fundamental step in most Bayesian phylogenetic analyses. We need a full tutorial for this.

[rbouckaert]

A tutorial for nested sampling would be good.

Note that there is already documentation on path sampling: http://beast2.org/path-sampling/ and http://beast2.org/2014/07/14/path-sampling-with-a-gui/, and path sampling is part of the species delimitation tutorial.

[rbouckaert]

There is a model selection tutorial for nested sampling: https://taming-the-beast.org/tutorials/NS-tutorial/