checks for sum_constraint leads to errors in utils.py

[sqbl]

As title says.

During instantiation of a constraint-class, all types of constraints are added (e.g. constraints.py l. 30). This means that when utils.py (e.g. l 296) checks for hasattr(constraint, "sum_equals") it will always get a "True".

Solutions (?): Make a factory-class for constraints that only add attributes based on input from user or change the checks for sum_constraint in utils.py (?)

Finally, we should likely add some tests to ensure that "old" constraints are still working

[SRFU-NN]

If there are no sum constraints, constraint.sum_equal is an empty list, right? So could we "just" change if hasattr(res.constraints, "sum_equals"): to if len(res.constraints)>0: in the lines 276 and 296? They are the only places in the repo where "sum_equals" is used.

But we absolutely need tests. It seems like the problem is localized to expected_minimum(), which might explain why tests didn't catch it.

[sqbl]

Fixed with PR #308