Add functionality for setting custom thresholds

[matthiasschaub]

Thresholds are used during the computation of indicators to decide the quality of the result. This can be simple values or more sophisticated functions. Those are currently defined either in the indicator.py or in the metadata.yaml file for each indicator.

It makes sense to set different thresholds for multiple instances of an indicators (e.g. same indicator for different layer or regions).

Implement custom thresholds:

• GHSPOP Indicators

Ideally allow to define parameters in threshold function.

[matthiasschaub]

Should there be a default threshold? Should this be definied at the indicator level or on the layer level or indicator+layer combination level?

[matthiasschaub]

Closing this for now.

Thresholds for each indicator are often implemented has function. Therefor to change the threshold one needs to understand implementation details of the indicator. This is not very user friendly.

A workaround is to request the indicator using the OQT API and then using the provided data to reclassify with a custom threshold.