Downcasting reworked to allow OFF and AUTO modes

example/conversion_factor.cpp

@@ -27,7 +27,7 @@
 namespace {
 
 template<units::Quantity Target, units::Quantity Source>
-  requires units::equivalent_dim<typename Source::dimension, typename Target::dimension>
+  requires units::equivalent<typename Source::dimension, typename Target::dimension>
 inline constexpr std::common_type_t<typename Target::rep, typename Source::rep> conversion_factor(Target, Source)
 {
   // get quantities looking like inputs but with Q::rep that doesn't have narrowing conversion