refactor: Refactorized whole project structure

source/functions/CMakeLists.txt

@@ -0,0 +1,11 @@
+
+target_sources(${PYG2O_MODULE_NAME}
+    PRIVATE
+        python/functions.cpp
+        squirrel/functions.cpp
+)
+
+target_include_directories(${PYG2O_MODULE_NAME}
+    PRIVATE
+        ${CMAKE_CURRENT_LIST_DIR}/
+)