Add possiblity for logical max-value choice in saturation

When calculating the saturation of LULC (https://github.com/GIScience/ohsome-quality-analyst/blob/main/workers/ohsome_quality_analyst/ohsome/layer_definitions.yaml#L303-L318) the computed saturation is useful but there is a logical value (100% of the area should be covered) available. This is much simpler to compute: `current-LULC-area/area of the aoi`. The user can currently compute this value herself (the current LULC area is returned from the API), but we may want to keep this in mind when creating a LULC-report.

Yet, the functionality of this computation may be useful for other indicators as well e.g. the highway length in relation to the highway length predicted by a model.

Please add the discussion-tag.