problems: make the name() methods static

this allows to retrieve the name of the problem before it is
instantiated. this is required to be able to print the "Initializing
problem" message at the correct point (i.e., before instantiating the
problem).

examples/tutorial1problem.hh

@@ -192,8 +192,8 @@ public:
     }
 
     //! Specifies the problem name. This is used for files generated by the simulation.
-    const char *name() const
-    { return "tutorial_coupled"; }
+    static std::string name()
+    { return "tutorial1"; }
 
     //! Returns the temperature at a given position.
     template <class Context>