[ENH] forecast broadcaster for applying univariate forecasters to multivariate time series

[TonyBagnall]

resolves #3004

draft version, will need to sort out testing

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[MatthewMiddlehurst]

Needs some tests IMO as we dont really have anything that does multivariate currently in forecasting. Possibly a more descriptive name.

[TonyBagnall]

Needs some tests IMO as we dont really have anything that does multivariate currently in forecasting. Possibly a more descriptive name.

any ideas on the name? still draft, not written tests yet, need to decide on whether output polymorphism is an issue or not

[TonyBagnall]

oh, not draft!

[MatthewMiddlehurst]

Maybe just MultivariateBroadcastForecaster