box models: change default differentiation method to forward differences

In my experience it never made a difference stability-wise but
performance-wise the speedup is almost factor 2 for local jacobian
assembly.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6078 2fb0f335-1f38-0410-981e-8018bf24f1b0

dumux/boxmodels/common/boxlocalresidual.hh

@@ -489,7 +489,7 @@ protected:
             int i = fvElemGeom_().subContVolFace[k].i;
             int j = fvElemGeom_().subContVolFace[k].j;
 
-	    PrimaryVariables flux;
+            PrimaryVariables flux;
             Valgrind::SetUndefined(flux);
             this->asImp_().computeFlux(flux, k);
             Valgrind::CheckDefined(flux);

dumux/boxmodels/common/boxpropertydefaults.hh

@@ -176,8 +176,8 @@ public:
  */
 SET_TYPE_PROP(BoxModel, JacobianAssembler, Dumux::BoxAssembler<TypeTag>);
 
-//! use central differences to calculate the jacobian by default
-SET_INT_PROP(BoxModel, NumericDifferenceMethod, 0);
+//! use forward differences to calculate the jacobian by default
+SET_INT_PROP(BoxModel, NumericDifferenceMethod, +1);
 
 //! do not use hints by default
 SET_BOOL_PROP(BoxModel, EnableHints, false);