kmeans: use norm as default metric

this avoids relying on 'point' being the coordinate type

kmeans.hpp

@@ -75,7 +75,7 @@ namespace util {
             std::forward<InputT>  (src),
             std::forward<OutputT> (dst),
             [] (auto a, auto b) {
-                return distance (a, b);
+                return norm (a - b);
         });
     }