Provide a virtual destructor

dune/tectonic/nicefunction.hh

@@ -10,6 +10,8 @@
 namespace Dune {
 class NiceFunction : public VirtualFunction<double, double> {
 public:
+  virtual ~NiceFunction() {}
+
   virtual double leftDifferential(double s) const = 0;
   virtual double rightDifferential(double s) const = 0;
 };