From the Project Spec 5, implement a SizedContainerTimeSeriesInterface ABC.
This is also an ABC, but its one that inherits from the TimeSeriesInterface ABC.
This ABC now adds in the methods you left out in the TimeSeriesInterface which pertain to its "containerness": the notions of indexing, length, and returning arrays in addition to iterators.
From the Project Spec 5, implement a
SizedContainerTimeSeriesInterface
ABC.