[ENH] add a difference transformer to series transformations

[TinaJin0228]

Reference Issues/PRs

fixes #1553

What does this implement/fix? Explain your changes.

This implements a transformer that computes the n-order differences of a time series.

The input time series is expected to have a NumPy inner-type, and the transformation is performed using numpy.diff().

To preserve the original shape of the series along the time axis, the first order element(s) of the output are filled with NaN.

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

No. It only depends on NumPy.

Any other comments?

No.

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I have added the following labels to this PR based on the title: [ $\color{#FEF1BE}{\textsf{enhancement}}$ ].
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[TinaJin0228]

It seems that one of the automated checks failed with errors indicating it couldn't find the merge base or my commit SHA:

Warning: Unable to find merge base between 45d0390d1a441264943f8963547409bb36935d2d and 8f78ec98e57b49640bd59f5570f35efb9fea7e67
Error: Unable to locate the commit sha: 45d0390d1a441264943f8963547409bb36935d2d
Error: Please verify that the commit sha is correct, and increase the 'fetch_depth' input if needed
Warning: fatal: bad object 45d0390d1a441264943f8963547409bb36935d2d
Warning: If this pull request is from a forked repository, please set the checkout action `repository` input to the same repository as the pull request.
Warning: This can be done by setting actions/checkout `repository` to ${{ github.event.pull_request.head.repo.full_name }}

[MatthewMiddlehurst]

Thanks, few comments.

[TinaJin0228]

I have made som modifications according to your comments. @MatthewMiddlehurst

[TinaJin0228]

I have made som modifications according to your comments. @MatthewMiddlehurst

@MatthewMiddlehurst I am wondering if you could give it a quick look when you get a chance? Thanks!

[MatthewMiddlehurst]

LGTM for the most part, just one documentation bit.

[TonyBagnall]

lgtm, thanks

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