Updating `get_seq_iter` and `get_buffered_seq_iter`

Renaming the following:
    `get_seq_iter` -> `get_seq_iter_from_file`
    `get_buffered_seq_iter` -> `get_buffered_seq_iter_from_file`
    `seq_iter` -> `get_seq_iter`
    `buffered_seq_iter` -> `get_buffered_seq_iter`

As a result, `get_seq_iter` and `get_buffered_seq_iter` now take a
collection of file objects as the first argument, rather than a single
file.

Updating all calls to `get_seq_iter` and `get_buffered_seq_iter`
throughout the package accordingly.