Use consistently first order interpolation

src/field/field3d.cxx

@@ -501,7 +501,7 @@ void Field3D::setBoundaryTo(const Field3D& f3d) {
         // Interpolate midpoint value in f3d
         const BoutReal val = pnt.interpolate_sheath_o1(f3d);
         // Set the same boundary value in this field
-        pnt.dirichlet_o2(*this, val);
+        pnt.dirichlet_o1(*this, val);
       }
     }
   }