Follow-up EstimatorReport: add an ADR for the design of EstimatorReport

[glemaitre]

We could write an ADR for the EstimatorReport to document some design choices and some additional vision. It would be useful for future maintenance and subsequent design.

[augustebaum]

Questions which arose during the presentation yesterday, along with what I understood the answers to be:

Why not compute and cache everything upfront, instead of exposing an explicit reporter.cache_predictions() method?

Answer: Just in case the estimator takes a long time to do things

Why do the metrics methods return pandas DataFrames?

Answer: So that the DFs can be concatenated and chained (e.g. using pandas' .plot)

Why accessors rather than hard-coding?

Answer: Accessors fit the problem better

[glemaitre]

Thanks @augustebaum, I'll integrate those points.