maths: add maxmin query

maths.hpp

@@ -645,6 +645,19 @@ namespace cruft {
     min (std::array<ValueT,CountV> &&) = delete;
 
 
+    ///------------------------------------------------------------------------
+    /// Returns an ordered pair where the elements come from the parameters.
+    template <typename ValueT>
+    std::pair<ValueT,ValueT>
+    maxmin (ValueT a, ValueT b)
+    {
+        if (a >= b)
+            return { a, b };
+        else
+            return { b, a };
+    }
+
+
     ///////////////////////////////////////////////////////////////////////////
     // Limiting functions