Create a report on the quality of the `fit`

Ensemble based assimilation can be modularized, see #5628
Therefore a sensible step towards an analysis report on the history match #5556 is to first report on the properties of the `fit`.
The `fit` is a statistical procedure, either parametric or semiparametric, and there are (usually) statistical methods to report on the quality of such procedures.
This is the case unless we do some crazy things with the `fit` afterwards.
This is a hint towards not doing crazy things with the `fit`afterwards.

Start by reading the context in #5628 and the history-match report request in #5556. Identify what properties of the `fit` need to be reported and which statistical methods are appropriate; the issue is complete when a report covering the agreed quality measures is produced.

Written by the indexing model from the issue text.