refactor: math_core.h moved back to the utility directory

src/core/CMakeLists.txt

@@ -43,7 +43,6 @@ add_library(
     include/mp-units/bits/get_associated_quantity.h
     include/mp-units/bits/get_common_base.h
     include/mp-units/bits/magnitude.h
-    include/mp-units/bits/math_core.h
     include/mp-units/bits/quantity_cast.h
     include/mp-units/bits/quantity_concepts.h
     include/mp-units/bits/quantity_point_concepts.h

src/utility/CMakeLists.txt

@@ -24,5 +24,6 @@ cmake_minimum_required(VERSION 3.19)
 
 add_units_module(
     utility DEPENDENCIES mp-units::core mp-units::isq mp-units::si mp-units::angular
-    HEADERS include/mp-units/chrono.h include/mp-units/math.h include/mp-units/random.h
+    HEADERS include/mp-units/bits/math_angular.h include/mp-units/bits/math_core.h include/mp-units/bits/math_si.h
+            include/mp-units/chrono.h include/mp-units/math.h include/mp-units/random.h
 )