[ENH] Add Inverse Transform to differencing transformer

[alexbanwell1]

Reference Issues/PRs

Builds on #2987

What does this implement/fix? Explain your changes.

Adds undifferencing transform to DifferencingTransformer

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

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[alexbanwell1]

It appears that storing X from transform isn't allowed either - the tests are failing because I add that attribute!