[ENH] Add AutoETS Algorithm

[alexbanwell1]

Reference Issues/PRs

What does this implement/fix? Explain your changes.

Adds AutoETS

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No

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[alexbanwell1]

Branch based off AutoARIMA branch, to avoid file conflicts when that is merged. Need to work out how to deal with ZeroDivisionErrors in the loss function, as currently number is not used in the main AutoETS function as it needs to catch any ZDEs