Remove constexpr from non-const methods

Non-const constexpr is not supported by gcc-4.9

dune/solvers/common/copyorreference.hh

@@ -41,7 +41,7 @@ public:
     return t_;
   }
 
-  constexpr Type& get()
+  Type& get()
   {
     return t_;
   }
@@ -72,7 +72,7 @@ public:
     return *t_;
   }
 
-  constexpr Type& get()
+  Type& get()
   {
     return *t_;
   }