xo-unit: + quantity printing + tests for mass units

include/xo/unit/quantity_iostream.hpp

@@ -0,0 +1,31 @@
+/** @file quantity_iostream.hpp
+ *
+ *  Author: Roland Conybeare
+ **/
+
+#pragma once
+
+#include "quantity.hpp"
+#include "natural_unit_iostream.hpp"
+
+namespace xo {
+    namespace qty {
+        template <
+            typename Repr = double,
+            typename Int = std::int64_t,
+            natural_unit<Int> NaturalUnit = natural_unit<Int>(),
+            typename Int2x = detail::width2x<Int>
+            >
+        inline std::ostream &
+        operator<< (std::ostream & os,
+                    const quantity<Repr, Int, NaturalUnit, Int2x> & x)
+        {
+            os << x.scale() << x.abbrev();
+            return os;
+        }
+
+    } /*namespace qty*/
+
+} /*namespace xo*/
+
+/** end quantity_iostream.hpp **/