GPMixin: Fix wrong IPOPT option when using minimization goals

We check if the DAE (the set of equality constraints) is linear, and tell IPOPT if we find that is the case for performance reasons. Any user added equality constraints are assumed to be linear as well, but for those added by GoalProgrammingMixin for minimization goals that is not necessarily the case. We should therefore make sure to not tell IPOPT that it can assume the equality constraints to be linear when adding such a constraint.

Note that this only applies to minimization goals, and only when fix_minimized_values is set to True (which is the default for IPOPT/BONMIN, but not for other solvers).

Closes #1103 (closed)