Add jackknife function (see #66) in the sandbox module (see #67)

`jackknife`: repeatedly iterate over seq, each time omitting a successive element

This is the first addition to the sandbox.   The purpose and usage of the sandbox still needs to be documented.  Recall that one of the goals is to engage new developers by making it easier to contribute new functions that don't yet have a suitable home.

The goal for `jackknife` is to eventually be in a `sampling` module, which may end up being a separate project.