Adding functions to `seqmod` module.

Adding `longest_reading_frames` and `translate_longest_reading_frames`
functions to `seqmod` module. The former returns a generator that yields
the longest reading frame found in each input sequence. The latter
returns a generator of the translated amino acid sequences encoded by
the longest reading frame.

Adding (weak) tests to `test_seqmod.py`.
Rearranging `seqops` modules.

Moving summarizing functions into new `seqops.seqsum` module. These
include `summarize_distances` and `summarize_longest_read_lengths`.
Updating test scripts and imports accordingly.
Adding `-g/--count-gaps` option to `seqaid.py`.
Overhauling `seqvet.py` script.

Adding new options and grouping all options into argument groups.  Major
new features include the ability to compare sequence distances based on
the translated amino acid sequences encoded by the longest reading frame
found in each sequence. Also, there is an option to report the longest
reading frame lengths of each sequence. There are several new
"Translation Options", which control how the longest reading frame of
each sequence is identified.