ENH: Implement selection sort

[seberg]

We currently have a small implementation of a merge sort algorithm only. We would like to also provide a selection_sort implementation for comparison.

For information about selection sort, please see: https://en.wikipedia.org/wiki/Selection_sort

Working on this issue

Implementing bubblesort consists of the following things:

• Find the existing selection_sort stub and replace it with your implementation.
• Add documentation (you can look at merge_sort for inspiration).
• Add some tests by expanding test_selection_sort in test_sorting.py.

Once you have made your changes, and committed them to a new branch, you can create a PR!