Include guards

src/properscalarincreasingconvexfunction.hh

+#ifndef PROPER_SCALAR_INCREASING_CONVEX_FUNCTION_HH
+#define PROPER_SCALAR_INCREASING_CONVEX_FUNCTION_HH
+
 class ProperScalarIncreasingConvexFunction {
 public:
   virtual double operator()(const double s) const = 0;
@@ -22,3 +25,5 @@ class TrivialFunction : public ProperScalarIncreasingConvexFunction {
 
   double rightDifferential(const double) const { return 0; }
 };
+
+#endif