Proposal for MultiPropertyModel

[stefaanhessmann]

I tested this on QM9 for single and multiple properties and it seems to run well.
Arguments can be passed as list (for multi) and as the single-prop arguments, so the SinglePropertyModel could be replaced by this.

[stefaanhessmann]

change of log titles prevents errors with property names that contain "_" because the split("_") of {pname}_{metric_name} will fail if pname also contains a "_". Therefore I changed it to {pname}:{metric_name}