Updating `sequtils` module.

Changing `dataio` module import to an import of the `seqsift` package.
Fixing `FileFormats._extenstions` attribute.

Adding missing `self.`.
Updating `dataio` module.

Fixing error in `seq_iter` function (changing `file_obj` to
`file_objs`). Also, adding `get_seq_batch_iter_from_files` function.
Updating `seq-file_slice.py`.

Getting initial implementation up and running.