Invalid objectiveBound statistic

[Dekker1]

I found that the FlatZinc interface will often output objectiveBound=inf as a statistic. However, this is parsed incorrectly by the MiniZinc driver/compiler, which will interpret this as an identifier inf that doesn't exist.

Currently, this will make MiniZinc output invalid output, but in the future this will likely produce a warning and the statistic will be ignored.

This should also be addressed in the solver interface. The easiest solution would be for the output to be valid DZN: objectiveBound=infinity (infinity and -infinity are the designated constants in the MiniZinc/DZN language). Otherwise, the interface could also just choose to not output this statistic when the bound is infinity, since there is no bound on the objective.