PDF extensions

[suchanj]

Closes #16, #30

[suchanj]

Note: The PDF's from UED and XRD slightly differ. Verify that this is expected behavior based on different signal types.

[suchanj]

Ok so I have:

• Added the optionality to choose rPDF (default), PDF or 1/rPDF via the --pdf-mode flag (and updated plotting, export and README)
• Implemented the PDF for XRD in the same way as in UED
• Simplified the code by moving the calc_single, calc_difference, calc_trajectory, calc_ensemble functions into the BaseDiffractionCalculator class, which is inherited by XRD and UED
  @JanosJiri its quite a lot of changes but could you take a look once you're available? no rush

[JanosJiri]

🔥🔥🔥Awesome work!!! 🔥🔥🔥

Just some small modification suggested.