MFPADs with polarization calculations

[phockett]

Notes

• Implemented and tested as of 5654489
• Docs:
  • E-fields with demo https://epsproc.readthedocs.io/en/3d-afpad-dev/demos/Epol_class_demo_docs_030324-tidy.html#Using-polarized-fields-for-PAD-calculations
  • Updated frame definitions: https://epsproc.readthedocs.io/en/3d-afpad-dev/special_topics/frame_definitions_docs_130323.html
  • General PAD demos: (in progress)

Issues

• In MF-BLM testing for some cases low cross-sections may result in unphysical 𝛽𝐿,𝑀 results. These currently require manual checking of the corss-sections, but should be automatically flagged/filtered in future.