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

example/total_energy.cpp

@@ -20,11 +20,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/physical/natural/constants.h>
-#include <units/physical/si/energy.h>
-#include <units/physical/si/momentum.h>
-#include <units/physical/si/speed.h>
-#include <units/physical/si/constants.h>
+#include <units/physical/natural/natural.h>
+#include <units/physical/si/si.h>
 #include <units/math.h>
 #include <iostream>