Adding functions to `seqmod` module.

Adding `reverse_complement_to_first_seq` and
`reverse_complement_to_longest_reading_frame` to `seqmod` module.
Adding associated tests to `test_seqmod.py`.
Updating `seqstats` module.

Adding `get_distances` function, which gets the distance and the
reverse-complement distance between two sequences. Updating
`pairwise_distance_iter` and `sample_distance_iter` to use
`get_distances`.
Working on adding `get_longest_reading_frames`.

Working on adding `get_longest_reading_frames` function to `sequtils`
module. It is not working yet.
More work on `get_longest_reading_frames`.

Still not working.