 <code>thin_percent</code> is not a
perfect multiple of the number of iterations per chain - this causes the
thinning to be “quasi-evenly” spaced. As this function is only ever used
in code development and not final products, this should not be an issue
(<a href='https://github.com/bstaton1/postpack/issues/13'>#13</a>).</li>
<li>Reversed <code>thin_percent</code> to be <code>keep_percent</code>,
and implemented throughout.</li>
<li>Added a <code>keep_iters</code> argument – now users can choose to
kee