use finishInit() properly for all problems

this means that all code which could potentially throw an exception is
moved to this method(). (In particular FluidSystem::init() proved
troublesome in the past.) Besides avoiding segmentation the faults
which stem from exceptions thrown in constructors, this also has the
advantage that simulations which spend a noticable amount of time to
initialize stop at the "correct" place, i.e. after the "Finish init of
the problem" message was printed by the simulator...

examples/problems/fractureproblem.hh

@@ -217,7 +217,15 @@ public:
      */
     FractureProblem(Simulator &simulator)
         : ParentType(simulator)
+    { }
+
+    /*!
+     * \copydoc FvBaseProblem::finishInit
+     */
+    void finishInit()
     {
+        ParentType::finishInit();
+
         eps_ = 3e-6;
         temperature_ = 273.15 + 20; // -> 20°C