[filters] Improve performance of median filter by using nth_element

[mvieth]

nth_element is faster than partial_sort, and should be used for median finding

[Morwenn]

Suggestion:

auto middle_it = vals.begin () + vals.size () / 2;
std::nth_element (vals.begin (), middle_it, vals.end ());
float new_depth = *middle_it;

[mvieth]

Suggestion:

auto middle_it = vals.begin () + vals.size () / 2;
std::nth_element (vals.begin (), middle_it, vals.end ());
float new_depth = *middle_it;

@kunaltyagi @SergioRAgostinho What's your opinion on this? Should I do this?

[SergioRAgostinho]

It's up to you. I'm okay with both.

[mvieth]

I added the suggestion. The failed test seems unrelated.