Add quickselect

[camilne]

Issue title

Add quickselect

Issue description

The quickselect algorithm is used to find the k-th smallest element in an unordered sequence.

Details:
Side effects: The sequence becomes partially sorted.
Average-case Time: O(n)
Worst-case Time: O(n^2) --very unlikely
Space: O(n)?, where n is the recursion depth

Checklist

• Check for existing issue
• Check for existing pull request