[FEATURE] Reorganize code in source files like main_model_impl.hpp to polish away traces of 'organic growth'

[Jerry-Jinfeng-Guo]

In C++ source files like main_model_impl.hpp, the code is appended every time new features are added. For example, you could see multiple switches of public, private and protected back and forth. This made the code less readable and coherent. In the feature request, an investigation is proposed to look for such incoherency and correct them.