Build build_ensemble() function

[saraloo]

Name tbc but a function that builds an ensemble forecast? Do we need this wrapper if one already exists in hubverse?

What nuances do we need here?

[saraloo]

Exists within current get_fcast() function. Is it too complicated to use the Hubverse function so that we can allow flexibility to change the ensembling method or weights etc?

[CyGei]

The ensemble method from distributional is very flexible and integrates nicely. If we eventually build non-fable models that can't fit with the current framework we can see alternative options.