refactor(example): partial headers replaced with the isq header

example/storage_tank.cpp

@@ -31,8 +31,7 @@ import mp_units;
 #else
 #include <mp-units/format.h>
 #include <mp-units/math.h>
-#include <mp-units/systems/isq/mechanics.h>
-#include <mp-units/systems/isq/space_and_time.h>
+#include <mp-units/systems/isq/isq.h>
 #include <mp-units/systems/si/si.h>
 #endif