Fix: use ref

dune/tectonic/ellipticenergy.hh

@@ -32,7 +32,7 @@ template <int dim> class EllipticEnergy {
   }
 
   // returns false if the direction is tangential
-  bool descentDirection(SmallVector const x, SmallVector &ret) const {
+  bool descentDirection(SmallVector const &x, SmallVector &ret) const {
     // Check the squared norm rather than each component because
     // complementaryProjection() divides by it
     if (x.two_norm2() == 0.0) {