Sequence types need to be polymorphic

[brimoor]

In eta.core.types, we have types like FileSequence that represent a sequence of files indexed by one numeric parameter, like /path/to/files/%05d.ext.

Currently, if one wants to declare that one is writing a sequence of, say, VideoLabels files, one must define a new concrete type VideoLabelsSequence to capture that.

It would be preferable to have a polymorphic type, say, Sequence, and the ability to write something like eta.core.types.Sequence<eta.core.types.VideoLabels>, which declares that you are expecting a sequence of VideoLabels files, without having to introduce a new concrete type in eta.core.types.