refactor(utils): Fix include paths in field_iteration.hpp

Normalize include paths to use full openpfc/ prefix:

- core/box3d.hpp → openpfc/core/box3d.hpp
- multi_index.hpp → openpfc/multi_index.hpp

Motivation: align with project-wide include path conventions,
improve build consistency across different include directories.

Author: ahojukka5

include/openpfc/utils/field_iteration.hpp

@@ -34,8 +34,8 @@
 #ifndef PFC_UTILS_FIELD_ITERATION_HPP
 #define PFC_UTILS_FIELD_ITERATION_HPP
 
-#include "core/box3d.hpp"
-#include "multi_index.hpp"
+#include "openpfc/core/box3d.hpp"
+#include "openpfc/multi_index.hpp"
 #include <functional>
 
 namespace pfc {