refactor: physical replaced with isq

Refers to #249

example/alternative_namespaces/density.h

@@ -2,7 +2,7 @@
 
 #pragma once
 
-#include <units/physical/si/density.h>
+#include <units/isq/si/density.h>
 
 namespace units{
 namespace experimental{
@@ -10,7 +10,7 @@ namespace experimental{
 namespace density {
 
 template<typename Rep = double>
-using kg_per_m3 = units::physical::si::density<units::physical::si::kilogram_per_metre_cub, Rep>;
+using kg_per_m3 = units::isq::si::density<units::isq::si::kilogram_per_metre_cub, Rep>;
 
 }