Bug fix: multiply derivative by coefficient as well

dune/tectonic/nicefunction.hh

@@ -55,14 +55,14 @@ class RuinaFunction : public NiceFunction {
     if (eta * s <= rho)
       return 0;
 
-    return normalStress * (a * std::log(s) + mu);
+    return coefficient * normalStress * (a * std::log(s) + mu);
   }
 
   double virtual rightDifferential(double s) const {
     if (eta * s <= rho)
       return 0;
 
-    return normalStress * (a * std::log(s) + mu);
+    return coefficient * normalStress * (a * std::log(s) + mu);
   }
 
 private: