refactor: quantity_io added

src/include/units/quantity.h

@@ -314,21 +314,6 @@ public:
 #endif
 
   [[nodiscard]] friend constexpr bool operator==(const quantity& lhs, const quantity& rhs) = default;
-
-  template<class CharT, class Traits>
-  friend std::basic_ostream<CharT, Traits>& operator<<(std::basic_ostream<CharT, Traits>& os, const quantity& q)
-    requires requires { os << q.count(); }
-  {
-    if(os.width()) {
-      // std::setw() applies to the whole quantity output so it has to be first put into std::string
-      std::basic_ostringstream<CharT, Traits> s;
-      detail::to_stream(s, q);
-      return os << s.str();
-    }
-
-    detail::to_stream(os, q);
-    return os;
-  }
 };
 
 // CTAD