[ENH] Add Basic ARIMA model

[alexbanwell1]

Reference Issues/PRs

Split out of #2828
Basis for #2838

What does this implement/fix? Explain your changes.

Adds basic ARIMA forecaster, which fits phi, theta (and optionally c) parameters, but takes in the model to use e.g. ARIMA(p,d,q) and an option to include a constant term.

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

No

Any other comments?

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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 start, some tidying up needed and we should think about utils structure

[TonyBagnall]

this will need some testing, esp for _calc_arima.

Actually think _calc_arima hould be called _arma_forecast, and might need refactoring for easier use with predict