Add `sc.average` helper function.

[arm61]

Might still need to divide the variance of the output by $\sqrt{N}$, based on the scipp.mean docs https://scipp.github.io/generated/functions/scipp.mean.html.

[arm61]

Met by my favourite error 😢

[nvaytet]

@arm61 I know it's been a while since you posted, but do you need some help fixing your favourite error?

[arm61]

I went in a huff and just implemented it in my code myself using line 143 from this PR.