mps_loader: MPS.add_range - mislabeled attribute

[dansplain]

https://github.com/jmaerte/pysmps/blob/aacdfc26959a0b267b331860e1fb717d924b3a52/pysmps/mps_loader.py#L195C23-L195C23

Hello - Thank you for making this code available, it is extremely helpful. I think there may be an incorrectly labeled attribute, in particular in mps_loader.py on L195 should the attribute constraint be constraints, so that the line reads:

elif self.constraints[constr]["type"] == "E":