BUG: wrong search for pivot in `quicker_sort`

This PR corrects an error in the search for the existence of a distinct
pivot for sorting in the `quicker_sort` routines. This could lead to
`quicker_sort` wrongly believing that all items in a subsequence are equal and
hence require no sorting, when only the first, second, middle and last items
are the same.

It also fixes the indentation in the `quicker_sort` part of `unwrap_2d_ljmu.c`

While this doesn't explain #835 and #902 on its own, it may be related.