[WIP] Tiered annotations

[hadware]

Old idea, finally took some time to implement it.

This is more or less an implementation of the Praat/Elan tiered structure. Here are a couple of key points for this

• There are two data structures TieredAnnotation and Tier. A TieredAnnotation is made out of Tiers
• The structure is basically and schematically a dict of dict : TieredAnnotation[str, Tier[Segment,Label]]
• A Tier is a thin wrapper over a timeline, used to keep store segments and take care of the cropping/support/etc-related operations
• The TieredAnnotation shouldn't have that many available methods other than:
  • accessing tiers and tiers information
  • cropping/extruding/etc (all just done by the underlying Tiers)
  • Iterating over segment
• Support for textigrid files parsing is optional, and added by running pip install pyannote.core[textgrid]. This installs textgrid-parser, my sly-based parser.

I'll follow up with a couple of questions to tune my implem to your preferences