Add validation for waste_outputs

[mabijkerk]

Background
In the model, the attribute waste_outputs can be specified for CHPs. It is used to make sure that the output of steam_hot_water is discounted in the marginal costs of the CHP. This is however only done for CHPs with a steam turbine, as is explained in the documentation.

Issue
Based on the node file on ETSource alone, it is hard to tell whether the waste_outputs attribute has been intentionally left out for a CHP, or whether it has been forgotten. If you don't thoroughly read the documentation, you might think that it has been forgotten.

Solution
Add validation for waste_outputs for CHPs. We could add a group that excludes a CHP from requiring the waste_outputs attribute. The validation would then fail if a CHP has the group AND waste_outputs, or when the CHP does not have the group NOR waste_outputs.