Flip signs in Richards FV local operator

Also remove the commented part on upwinding.

dune/dorie/model/richards/local_operator_FV.hh

@@ -139,10 +139,10 @@ public:
     const RangeU u_o = x_o(lfsu_o,0);
 
     // Finite difference of u between the two entities
-    RangeU dudn = (u_i-u_o) / distance;
+    RangeU dudn = (u_o-u_i) / distance;
 
     // Update gradient with gravity vector
-    dudn += _param->gravity() * normal_f;
+    dudn -= _param->gravity() * normal_f;
 
     // bind parameterization and retrieve functions
     _param->bind(entity_i);
@@ -199,13 +199,7 @@ public:
     }
 
     // Water flux in normal direction w.r.t the intersection
-    const auto water_flux_n = cond_f * dudn;
-
-    // // Upwinding
-    // if (water_flux_n > 0.)
-    //   water_flux_n = - cond_i*dudn;
-    // else
-    //   water_flux_n = - cond_o*dudn;
+    const auto water_flux_n = - cond_f * dudn;
 
     // Symmetric contribution to residual on inside element
     r_i.accumulate(lfsv_i, 0,  water_flux_n*volume_f );