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

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.