pml demo restructuring to modular files

would like to contribute to the restructuring issue #3076 by proposing to refractor the python pml demo into modular files.

Why :

Helps separate specific parts of the problem to make it easier to navigate
Introduces the idea to of using modules to solve complicated problems like the pml demo

I have created the code and tested it, I can attach it for your reference. Let me know if you find this helpful and if there are anything you would suggest to do differently.

Start by reading issue #3076 and reviewing the existing Python PML demo structure. Compare the proposed modular organization and the author's tested code when it is available, then confirm the demo still behaves as before after the restructuring.

Written by the indexing model from the issue text.