refactor: physical replaced with isq

Refers to #249

example/alternative_namespaces/force.h

@@ -1,7 +1,7 @@
 
 #pragma once
 
-#include <units/physical/si/force.h>
+#include <units/isq/si/force.h>
 
 namespace units{
 namespace experimental{
@@ -9,7 +9,7 @@ namespace experimental{
 namespace force {
 
 template<typename Rep = double>
-using N = units::physical::si::force<units::physical::si::newton, Rep>;
+using N = units::isq::si::force<units::isq::si::newton, Rep>;
 
 }