Updating `spreadsheet_iter` function.

Removing the creation of an unused dict in the `spreadsheet_iter`
function of the `parsing` module.
Adding test of no sorting.

Adding a test to ensure that when the sorting index is 0,
`dpp-msbayes.pl` keeps the summary statistic indices in the same order
as the taxa in the config file.

This confirms this behavior when for a single locus when the sorting
index is 0.
Adding another sorting test.

Adding test to confirm that for a single-locus analysis, when the
sorting index is 7, that all prior samples generated via
`dpp-msbayes.pl` are sorted by sequence divergence (pi_net).