E2309. Refactor Node model and its subclasses

[nehakale8]

A description of the changes proposed in the pull request.

• Method name changed to something more informative wherever necessary.
• Making sure all the methods in the subclass are uniformly implemented. Refactoring the code for such methods.
• Adding appropriate comments in existing code in each file, which makes it easier to understand the functionality.
• Creating a method to return true/false based on courses.private.
• Writing and updating tests for any updated methods.