Updating `average_number_of_pairwise_differences`.

Adding check to make sure at least two sequences were provided.
If not, `None` is returned.
Adding `EmptySeqCollectionError` class.

Currently not being used.
Fixing typos in `Worker` and `PaupWorker`.

Both worker classes had typos in their constructor (missing '.' and
',').
Overhauling and debugging `pymsbayes_utils` module.

A lot of work throughout the module to get it basic converting
of pyrad output to pymsbayes input working. Still untested.