Reorganized demo_pml Post-processing

[Hawraa17]

• Created separate functions to calculate P and Q for better readability and reuse
• No functional changes to solver logic
• Condensed print statements (check if it is ok to print Analytical and Numerical values with only 6 floating points)

[Hawraa17]

Wanted to tag you both to review the work if you have any comments @jhale @jorgensd

[jorgensd]

Please remove these files

[jorgensd]

@Hawraa17 : I had a look at your changes, and while doing that, i decided to rewrite some of the less explained components of the code: #4000

This change would separate P and Q into simple one-liners, as no marker or "averaging" is needed.