fix issue in Constraints.add_constraint

[peterstangl]

No description provided.

[DavidMStraub]

Why aren't you keeping the existing behaviour as default to preserve backward compatibility?

[peterstangl]

I think that's what I'm doing now with this PR using the argument parameter_object=False. I introduced an issue in a previous commit in PR #247 where I actually broke backward compatibility. The present PR should restore it.

[DavidMStraub]

Perhaps parameter_object might benefit from a different name that fits to the fact that it's a boolean, not a parameter object

[peterstangl]

Maybe something like is_parameter_constraint? 🤔 The main point is just that this should only be applied to parameter constraints and not to measurement constraints.