Define dataset types with rail.plotting, i.e., structures that can be used to make particular plots.

[eacharles]

Right now RailPlotter classes define a dict of types, using inputs: dict = {} and check that the data matches the types.

RailProjectDataExtractor classes define a different dict of types, also using inputs: dict = {} to check that that they are being called with the correct inputs, and return a dict of data, which should match RailPlotter.dict(), but that isn't enforced.

RailDatasetHolder classes define extractor_inputs, which should match RailProjectDataExtractor.inputs, and enforce that, but do not enforce that they return a particular type of output data.

It would be good the enforce this, and to use names to match the types, so that we can quickly see which RailPlotters go with which RailProjectDataExtractor