Use weighted aggregated_by in our time-related preprocessors

[schlunma]

Since iris==3.3.0, Cube.aggregated_by supports weights. This function is used very often in our time-related preprocessors (e.g., annual_statistics, monthly_statistics, daily_statistics, etc.), but currently does not use weights (=length of the time interval).

We could/should add weights to these preprocessors to make them more accurate (e.g., annual_statistics should consider different number of days for different months).

I'll try to tackle this for v2.8, but not sure if I can make it in time.

Relies on solving ESMValGroup/ESMValTool#2945