Adding `sequences_are_equal` function.

Adding `sequences_are_equal` function to `sequtils` module.
Adding test to `test_sequtils.py`.
Adding `get_duplicate_ids` function.

Adding `get_duplicate_ids` function to `seqstats` module.
Adding tests to `test_seqstats.py`.
Updating tests of `summarize_distances` function.

Adding additional assertions to tests of `SummarizeDistancesTestCase` in
`test_seqsum.py`.
Updating `FileFormats` class.

Updating `FileFormats` class. Adding `get_ext` method to get file
extension from format. Also updating class to handle gzipped file names.
Updating `dataio` module.

Updating `BufferedIter` class and adding `seq_iter` and
`buffered_seq_iter` functions to `dataio` module.

Updating `BufferedIter` to allow objects to be appended to an instance.

`seq_iter` and `buffered_seq_iter` are similar to `get_seq_iter` and
`get_buffered_seq_iter` functions, but take a collection of file paths
or streams as the first argument (rather than a single file).
Adding `duplicate_id_filter` function.

Adding `duplicate_id_filter` function to `seqfilter` module, and adding
associated tests to `test_seqfilter.py`.