Metrics as methods of models

[lars-reimann]

Is your feature request related to a problem?

In some sense metrics are a property of models (in connection with some data).

Desired solution

Think about whether it makes sense to have them as methods on models. This should be easy to implement once #60 is done.

Possible alternatives (optional)

No response

Screenshots (optional)

No response

Additional Context (optional)

If this gets implemented, #63 is no longer needed.