`KDTree`: Assertion failure with `numNeighbours` < 0

[weefuzzy]

It is possible to set the numNeighbours control of KDTreeClient to < 0. However, the algorithm itself explicitly expects this to be nonnegative, and calls asUnsigned. Presumably in release builds, this will instead wrap (badness). We should enforce this invariant in the interface by adding Min(0).