Expand docs: more info required about aggregating the causal impact over time

[drbenvincent]

For the interrupted time series docs here https://causalpy.readthedocs.io/en/stable/notebooks/its_pymc.html we have an example causal impact over time.

The docs then talk about aggregating these values over time (mean), reporting that value, but also giving a warning.

A good point was raised in the pymc discourse which indicates more information is needed here.

This issue could be closed by:

• expanding the discussion of reducing the causal impact over the whole post-intervention period into a single distribution.
• Talking about when and why different aggregation functions could be useful:
  • mean - if you have a step change in the post-intervention period. Would give you the mean increase per time period
  • sum - gives you the total effect. Note that this is equivalent to looking at the final point in the cumulative causal impact.