Decide on unsolvable default stats

E.g., what should the lower and upper bound be for an unsolvable problem.

From Florian Pommerening:
> This might have different answers for different parts of the code. For example, if the database allows to store a special "infinity" value, then this still doesn't answer the question what the json response should be. And if json returns some special string (say "infnity"), then the python API could still parse this and return something else (say float("inf")) when asked for a bound.