Updating `summarize_longest_read_lengths` function

The function now reports the length of the longest reading frame of the
current and reverse-complemented orientation for each sequence.
Updating and adding tests accordingly.
Updating `longest_reading_frames` function

Updating `longest_reading_frames` function of `seqmod` module to handle
cases when there is no reading frames for a sequence. It returns a copy
of the `SeqRecord` with an empty sequence.
Updating `seqvet.py` script.

Updating `summarize-reading-frame-lengths` option to handle the fact
that the `summarize_longest_read_lengths` function now reports lengths
for the current and reverse-complement orientation of each sequence.
This extra information is now written to file.