Be less explicit about the 2D setting

dune/tectonic/ellipticenergy.hh

@@ -294,7 +294,8 @@ void minimisationInitialiser(Functional const &J, Bisection const &bisection,
     tangentialMinimisation(J, x1, descDir1, bisection);
     double const Jx1 = J(x1);
 
-    SmallVector x2 = { -x1[0], -x1[1] };
+    SmallVector x2(0);
+    x2.axpy(-1, x1);
     SmallVector const descDir2 = { x2[1], -x2[0] };
     tangentialMinimisation(J, x2, descDir2, bisection);
     double const Jx2 = J(x2);