feat: two argument explicit cast support added

example/hello_units.cpp

@@ -37,7 +37,11 @@ int main()
   using namespace units::physical::si::literals;
   Speed auto v1 = avg_speed(220_q_km, 2_q_h);
   Speed auto v2 = avg_speed(si::length<international::mile>(140), si::time<si::hour>(2));
+#if DOWNCAST_MODE == 0
+  Speed auto v3 = quantity_cast<si::dim_speed, si::metre_per_second>(v2);
+#else
   Speed auto v3 = quantity_cast<si::metre_per_second>(v2);
+#endif
   Speed auto v4 = quantity_cast<int>(v3);
 
   std::cout << v1 << '\n';                             // 110 km/h

example/measurement.cpp

@@ -131,7 +131,11 @@ void example()
   const auto t = si::time<si::second, measurement<double>>(measurement(1.2, 0.1));
 
   const Speed auto v1 = a * t;
+#if DOWNCAST_MODE == 0
+  std::cout << a << " * " << t << " = " << v1 << " = " << quantity_cast<si::dim_speed, si::kilometre_per_hour>(v1) << '\n';
+#else
   std::cout << a << " * " << t << " = " << v1 << " = " << quantity_cast<si::kilometre_per_hour>(v1) << '\n';
+#endif
 
   si::length<si::metre, measurement<double>> length(measurement(123., 1.));
   std::cout << "10 * " << length << " = " << 10 * length << '\n';