Including Signal2D/ BaseSignal methods in Documentation

[CSSFrancis]

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@hakonanes had a good comment about including parent methods in the documentation in diffsims and it got me thinking should we include the Signal2D/ BaseSignal methods in the documentation?

I could see how adding the Signal2D methods might really crowd the namespace but it might also be good to have those methods included in the documentation so people don't have to go upstream to hyperspy. Maybe something like separate the methods such that they go:

DiffractionSignal Methods:

....


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Parent Methods: 
(Links to outside)
....