Relationship between topic (and aggregation or measurement type) and indicators

[matthiasschaub]

Currently, a topic definition always relates to one aggregation type (E.g. building-count -> /count)

A user should be able to select a topic and indicator without thinking about the aggregation type. Hence, if the user selects the topic building and the indicator mapping saturation the result should be the saturation curve over the number of buildings and the building area.

[matthiasschaub]

https://github.com/GIScience/ohsome-quality-analyst/pull/600