ice1e0

See https://github.com/mara/mara-db/issues/44 It does not require that https://github.com/mara/mara-db/pull/47 is merged. This PR can be merged idependently.

Term discussion: metric vs measure

3

in the schema definition the terms 'simple metric' and 'composed metric' are used (e.g. see code file [metric.py](https://github.com/mara/mara-schema/blob/master/mara_schema/metric.py)). Shouldn't this be renamed: * 'simple metric' to measure and * 'composed...