[FEATURE] Metric definition in PanopticaResult unintuitive

[Hendrik-code]

Is your feature request related to a problem? Please describe. Currently, we have to define each metric in the PanopticaResult object as well.

Describe the solution you'd like A way to automatically define that a metric can be computed instance-wise, and thus creating sq_{metric} and the other metrics in the result object automatically. Maybe instead of members, have a dictionary that maps from original Metric (i.e. IOU) object to the specific downstream metrics (i.e. sq_iou, ...)