added operator<< for quantity

include/emlabcpp/quantity.h

@@ -7,6 +7,10 @@
 #include <string>
 #include <type_traits>
 
+#ifdef EMLABCPP_USE_STREAMS
+#include <ostream>
+#endif
+
 #pragma once
 
 namespace emlabcpp
@@ -233,6 +237,14 @@ constexpr ValueType operator/( const ValueType val, const quantity< Derived, Val
         return val / *q;
 }
 
+#ifdef EMLABCPP_USE_STREAMS
+template < typename T, typename ValueType >
+inline std::ostream& operator<<( std::ostream& os, quantity< T, ValueType > q )
+{
+        return os << *q << T::get_unit();
+}
+#endif
+
 }  // namespace emlabcpp
 
 /// The quantity has defined partital specialization of std::numeric_limits,