Soften assertion; Add a note

dune/tectonic/myblockproblem.hh

@@ -130,8 +130,10 @@ template <class MyConvexProblemTypeTEMPLATE> class MyBlockProblem {
 
     Interval<double> D;
     psi.subDiff(0, D);
-    if (D[1] >= 0)
-      assert(false);
+    if (D[1] > 0) {
+      assert(false); // NOTE: Numerical instability can actually get us here
+      return 0;
+    }
 
     int bisectionsteps = 0;
     Bisection bisection(0.0, 1.0, 1e-12, true, 0);                      // TODO