Add forecast accuracy to gauges view Forecast accuracy is calculated as the maximum deviation of confidence interval bounds from the prediction within the next four days. The JSON-aggregate of reference water levels is moved into a sub-select to avoid having to add further independent columns to the GROUP BY clause of the main statement.

The gemma database schema requires PostgreSQL >= 11.

`dot.tmpl` is a template file for `postgresql_autodoc`
to be more similiar to UML and leave out some less useful labels.

See  `../docker/README.md`