Plot substeps (ecmwf#789)

* Create subplots with grouping by valid_time.

* Create histograms at substeps with grouping by valid_time.

* Make use of inference run config to distinguish between situations where all datapoints of a sample should be plotted or where sub-stepping is required.

* Add helper function to get values of keys from stream configs.

* Corrected loading of model config.

* Ruffed code and turning message-level on cartopy path to debug.

* Revisions following reviewer comments.

* fix histograms

* ruffed

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Co-authored-by: Ilaria Luise <iluise00@login05.leonardo.local>
Co-authored-by: ilaria luise <luise.ilaria@gmail.com>

Author: 3 people