Improve permutest.betadisper returned object

This PR includes commits that aim to bring the object returned by `permutest.betadisper()` up to a similar level as other `permutest()` methods, such as for `adonis()` or `cca()`.

`permutest.betadisper()` now returns `statistic` and `perm` containing the observed F and any group pairwise t statistics, and a matrix of permuted values of these statistics, one statistic per column.

Fills out the `permustats.permutest.betadisper()` method so it matches other methods.

`permustats()` is now used in the examples for `permutest.betadisper()` and hence the reference materials are updated too.