"Multi-annotations" for new data objects in Neo 0.5

[mdenker]

Neo 0.5 introduces a number of Neo objects that can contain more than one data item:

• AnalogSignal
• Event
• Epoch
  For example, an Event object may contain multiple recorded event times. However, annotations are limited to the Event object as a whole. Likewise, filtering only works on the complete event objects.

This epic develops the concept to

• provide annotations for individual data items in one such Neo object (called multi-annotations for now)
• and the mechanisms to efficiently work with such multi-annotations