Improved calculation of hyperdual sqrt

dune/elasticity/common/hyperdual.hh

@@ -460,7 +460,12 @@ hyperdual atan(hyperdual x)
 }
 hyperdual sqrt(hyperdual x)
 {
-	return pow(x,0.5);
+	hyperdual temp;
+	temp.f0 = std::sqrt(x.f0);
+	temp.f1 = 0.5*x.f1/temp.f0;
+	temp.f2 = 0.5*x.f2/temp.f0;
+	temp.f12 = 0.5*(x.f12 - 2.*temp.f1*temp.f2)/temp.f0;
+	return temp;
 }
 hyperdual fabs(hyperdual x)
 {