Fix sign of production rate controls.

In the Wells struct, production rate control targets must be negative
(and injection rate control targets are always positive).
In the WellsGroup classes, there are separate variables for injection
and production, and all rates are positive. Therefore, upon adding or
modification of a production rate control, the negated value must
be used.

opm/core/wells/WellsGroup.cpp

@@ -863,7 +863,7 @@ namespace Opm
             return;
         }
         // We're a producer, so we need to negate the input
-        double ntarget = target;
+        double ntarget = -target;
         
         double distr[3] = { 0.0, 0.0, 0.0 };
         const int* phase_pos = phaseUsage().phase_pos;