allow `mode="mutation"` for `genetic_relatedness_vector` (and make this default?)

[petrelharp]

AFAICT, like for divergence_matrix there is not an efficient algorithm of the same sort to calculate the site-mode value; rather, the natural corresponding thing is mutation-mode (https://github.com/tskit-dev/tskit/issues/2982). And, this should probably be the default mode, instead of "branch", to mirror other stats (at least, divergence_matrix does)?