Adding `is_sequential` method to `FileFormats`.
Adding `write_seqs_to_files` function to `dataio`.

Adding `write_seqs_to_files` function to `dataio` module. The
function writes sequences to one or more files on the fly.
Updating `seq-file-slice.py` script.

Updating script for more efficient handling of sequential file formats.
The previous use of `SeqIO.write` for all file formats meant temporarily
storing `SeqRecord` objects in temp files via `BufferedIter`.

This is necessary for non-sequential file formats, but not for
sequential formats. If dealing with a sequential format, the script now
uses `write_seqs_to_files` function to write the sequences to files "on
the fly," which is much more efficient.
Adding `PathExistsError` to `errors` module.
Updating `write_seqs_to_files` function.

Raising `PathExistsError` rather than `Exception` if output files
already exist.
Updating `seq-file-slice.py` script.

Adding `--force` option to the CLI that enables the overwriting
of existing output files. Also, updating the error messages when
`--force` is not in effect and the output files exist.