Add a naive implementation of the vp-tree data structure for nearest neighbor search
Remove recursion from vp-tree searching, add optimizations
Add comment to the VpTree class
Remove test data generator, belongs only to the performance branch
Merging vp-tree into main branch

Merge branch 'master' of https://github.com/karelp/ddogleg into karelp-master
Merge branch 'master' of github.com:lessthanoptimal/ddogleg into karelp-master
- adding vp-tree
- updating for changes in ddogleg