Fix type

dune/tectonic/samplefunctional.hh

@@ -137,7 +137,7 @@ void minimise(Functional const J, typename Functional::SmallVector &x,
 
   for (size_t step = 0; step < steps; ++step) {
     SmallVector descDir;
-    double linesearchp = J.descentDirection(x, descDir);
+    bool const linesearchp = J.descentDirection(x, descDir);
 
     if (descDir == SmallVector(0.0))
       return;