[BUG] Fix incorrect undifferencing in ARIMA

[alexbanwell1]

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What does this implement/fix? Explain your changes.

Fixes incorrect undifferencing in ARIMA, adds inverse transform to DifferenceTransformer

Does your contribution introduce a new dependency? If yes, which one?

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[TonyBagnall]

could you take out the inverse of difference from this PR and make a separate one? I'm not sure atm how to do this to be consistent with the general interface, so it wpould be better as a separate PR imo

[TonyBagnall]

needs refactoring for the new model

[TonyBagnall]

see previous, also need testing for the undifference function in the transformer