ENH: Add seq format that allows onset_sample + offset_sample

[NickleDave]

We have a 'Sequence' data type but no format class that corresponds to that data type.

If I have a bunch of Sequences and want to save each in a single file, there's no easy way to do that.

There's 'simple-seq', but it assumes we're working with onsets + offsets in seconds. I think it might be possible to work around this with kwargs to the __init__ but I'm not sure.

Would be nice to have a Sequence format with methods like to_file, from_file, etc.

Not sure if this should get added to the current Sequence class or if there should be a separate class in crowsetta.formats.seq instead.