Bugfix: incorrect specialization

Some compilers are more liberal than others…

miniapp/io.cpp

@@ -11,10 +11,12 @@
 
 // Let TCLAP understand value arguments that are of an optional type.
 
-template <typename V>
-struct TCLAP::ArgTraits<nest::mc::util::optional<V>> {
-    using ValueCategory = ValueLike;
-};
+namespace TCLAP {
+    template <typename V>
+    struct ArgTraits<nest::mc::util::optional<V>> {
+        using ValueCategory = ValueLike;
+    };
+}
 
 namespace nest {
 namespace mc {