[ENH] add BaseSeriesTransformer - Moving Average Transformer

[Datadote]

Reference Issues/PRs

• Part of [ENH] Smoothing filters as BaseSeriesTransformers #1920

What does this implement/fix? Explain your changes.

• Add smoothing filter MovingAverageTransformer
• Tests have been added to check calculations with window_size < 0, == 0, > 0

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

• No new dependencies. Imports libraries: numpy

Any other comments?

• There are different ways to implement simple moving average.
  x Basic version is "for loop + window calculation" across the the input array.
  x Implemented version: 1) precalculate a cumulative sum 2) vector subtraction + division afterwards
  x Pros: Faster than basic version. O(1) vs O(window_size)
  x Cons: Requires extra memory to cache the cumulative sum. O(n) vs O(1)

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I have added the following labels to this PR based on the title: [ $\color{#FEF1BE}{\textsf{enhancement}}$ ].
I have added the following labels to this PR based on the changes made: [ $\color{#41A8F6}{\textsf{transformations}}$ ]. Feel free to change these if they do not properly represent the PR.

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[MatthewMiddlehurst]

Will let @TonyBagnall comment on functionality, but looks fine to me.

[TonyBagnall]

LGTM, only thing I can think of is to check that window length is not longer than the series, but not a blocker