not negative rate_for_group_control

in WellsGroup::updateWellProductionTargets. The current implementation
of group control is allowed tiny over-producing of the group target
while it cause negative rate_for_group_control . When all the wells are
not under group controls, it can cause oscillation of the control mode
later.

Probably a better way is to do something when we see overproducing
happens.

opm/core/wells/WellsGroup.cpp

@@ -788,7 +788,7 @@ namespace Opm
         }
 
         // the rates left for the wells under group control to split
-        const double rate_for_group_control = target_rate - rate_individual_control;
+        const double rate_for_group_control = std::max(target_rate - rate_individual_control, 0.0);
 
         const double my_guide_rate = productionGuideRate(true);