Improve perfomance of median calculation in LMedS algorithm

Improve perfomance of median calculation in LMedS algorithm

* Use `nth_element()` to find the median instead of `sort()` in `LMeDSPointSetRegistrator::run()`

* Improves performance of this part of LMedS from `n log(n)` to `n` by avoiding doing a full sort.

* Makes LMedS 2x faster for 100 points, 4x faster for 5,000 points in `EstimateAffine2D()`.

* LMedS is now never more than 2x slower than RANSAC and is faster in some cases.