[FEATURE] PGM internal steps before GUI

[Jerry-Jinfeng-Guo]

At the moment PGM uses serialization to and from data. This will become the crucial bridge for potential graphical user interface (GUI) operating on top of PGM. However, there are still a couple steps need to happen internally (not necessarily code wise) before GUI can be started. The following growing list docs the topics in speak:

• Generic/custom attributes for components [https://github.com/[FEATURE] Add custom attributes for serialization #493]
• Workflow design
• TBD

Note: it is not in the scope of PGM to provide a GUI for users, but PGM can be used as a calculation core for other programs/GUIs