[ENH] add BaseSeriesTransformer - ExpSmoothingSeriesTransformer

[Datadote]

Reference Issues/PRs

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

What does this implement/fix? Explain your changes.

• Add smoothing filter ExpSmoothingSeriesTransformer (note abbreviated Exp)
• Tests check for different alpha, and window_size values

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

• No new dependencies. Imports libraries: numpy, typing

Any other comments?

• Arxiv paper reference has an error in the exponential smoothing equation. The decaying term should be based on s_j-1, and not t_j-1 . I'm not sure what the Springer reference has. I do not have access
  Paper: s_0 = t_0 and s_j = alpha * t_j + (1 - alpha) * t_j-1
  Corrected: s_0 = t_0 and s_j = alpha * t_j + (1 - alpha) * s_j-1
• Allows user to use "alpha" or "window_size". If "window_size" is specified, alpha is overwritten with 2. / (window_size + 1)
• For the reference paper link, I added an arxiv link to the paper for folks without access to Springer

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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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[TonyBagnall]

good spot on the typo, but I would prefer this to be axis = 1 as it is our standard

[TonyBagnall]

LGTM, thanks for the changes