Modify Annotation to allow for multiple seq

[NickleDave]

Stack module was added but yes, looks like we're not using it?

Not sure now if we should be. Leaving this open

[NickleDave]

Changing the issue name but bringing it back to life.

We want to be able to handle TextGrids with multiple tiers.
See discussion here:
pyOpenSci/software-submission#68 (comment)

Need to look at the right way to implement this.

Can we just allow Seq to be either a single Sequence or a list of Sequences?
Not clear to me how much hierarchy are allowed by Tiers in TextGrids.

[NickleDave]

Thinking about this more, a quick fix for now I think will be to do the following:

• add a new attribute seqs which is a Python tuple of crowsetta.Sequences
• for seq-like formats that only ever have a single seq
• for formats that can have multiple seqs per annotation file, such as TextGrid, they can be converted to an annotation with multiple sequences