refactor: reference specifiers are now variable templates

test/static/usc_test.cpp

@@ -126,7 +126,7 @@ static_assert(isq::mass(1 * lb_t) == isq::mass(12 * oz_t));
 static_assert(isq::pressure(1'000 * inHg) == isq::pressure(3'386'389 * si::pascal));
 
 // Temperature
-static_assert(isq::thermodynamic_temperature(9 * delta(deg_F)) ==
-              isq::thermodynamic_temperature(5 * delta(si::degree_Celsius)));
+static_assert(isq::thermodynamic_temperature(9 * delta<deg_F>) ==
+              isq::thermodynamic_temperature(5 * delta<si::degree_Celsius>));
 
 }  // namespace