Updating `dataio` module.

Adding `SeqFileIter` class for handling all parsing of sequence
generators. Making `get_seq_iter` and `get_seq_iter_from_file`
classmethods of `SeqFileIter`. However, I am leaving aliases for
`get_seq_iter` and `get_seq_iter_from_file` in the namespace of
`dataio`, because they are used throughout the package.
Adding functions to `seqstats` module.

Adding `get_seq_summary` and `get_seq_summaries_from_files` functions to
the `seqstats` module. Adding associated tests to `test_seqstats.py`.
Renaming `seq-file-slice.py` script.

Renaming `seq-file-slice.py` script as `seqbundler.py`.
Adding `seqsummary.py` script.