Add distance epilogue functionality for bruteforce knn

[benfred]

The mutual reachability code inside of cuml needs to be able to specify a distance epilogue, to modify the distances after the distance computation but before the select-k step. This change adds an optional function to do this.

[cjnolet]

Discussed offline but I want to mention it publicly- we are discussing moving over the mutual reachabiility computation from cuML and making it a first-class distance within bfknn (and nn-descent as that will be needed also) so that we can avoid having to expose any type of distance epilogue.

[benfred]

mutual reachability was directly added in #323 -