refactor: SI units do not depend on the entire ISQ anymore + header dependency compilation fixes

example/foot_pound_second.cpp

@@ -23,6 +23,7 @@
 #include <mp-units/format.h>
 #include <mp-units/systems/imperial/imperial.h>
 #include <mp-units/systems/international/international.h>
+#include <mp-units/systems/isq/mechanics.h>
 #include <mp-units/systems/isq/space_and_time.h>
 #include <mp-units/systems/si/unit_symbols.h>
 #include <iostream>

src/systems/si/include/mp-units/systems/si/units.h

@@ -22,7 +22,8 @@
 
 #pragma once
 
-#include <mp-units/systems/isq/isq.h>
+#include <mp-units/systems/isq/base_quantities.h>
+#include <mp-units/systems/isq/space_and_time.h>
 #include <mp-units/systems/si/prefixes.h>
 #include <mp-units/unit.h>
 

test/unit_test/static/international_test.cpp

@@ -22,6 +22,8 @@
 
 #include <mp-units/math.h>
 #include <mp-units/systems/international/international.h>
+#include <mp-units/systems/isq/mechanics.h>
+#include <mp-units/systems/isq/space_and_time.h>
 #include <mp-units/systems/si/constants.h>
 #include <mp-units/systems/si/units.h>
 

test/unit_test/static/quantity_test.cpp

@@ -22,6 +22,7 @@
 
 #include "test_tools.h"
 #include <mp-units/chrono.h>
+#include <mp-units/systems/isq/mechanics.h>
 #include <mp-units/systems/isq/space_and_time.h>
 #include <mp-units/systems/si/si.h>
 #include <limits>

test/unit_test/static/usc_test.cpp

@@ -20,6 +20,7 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#include <mp-units/systems/isq/mechanics.h>
 #include <mp-units/systems/isq/space_and_time.h>
 #include <mp-units/systems/si/unit_symbols.h>
 #include <mp-units/systems/usc/usc.h>