refactor: file tree with systems defninitions refactored to limit ODR issues

Relates to #32 as with this commit we created something like modules for derived quantities

test/unit_test/runtime/math_test.cpp

@@ -21,8 +21,7 @@
 // SOFTWARE.
 
 #include "units/math.h"
-#include "units/physical/si/area.h"
-#include "units/physical/si/volume.h"
+#include "units/physical/si/si.h"
 #include <catch2/catch.hpp>
 
 using namespace units;