Feat: add field extraction for test particles

[henry2004y]

• Add EM field extraction along the test particle trajectory
• Add tests
• Add pitch angle calculation

[codecov]

Codecov Report

Attention: Patch coverage is 63.88889% with 26 lines in your changes missing coverage. Please review.

Project coverage is 55.29%. Comparing base (1b60f3b) to head (86dcbe2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/flekspy/tp/test_particles.py	62.31%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
- Coverage   55.75%   55.29%   -0.47%     
==========================================
  Files          12       12              
  Lines        1693     1727      +34     
==========================================
+ Hits          944      955      +11     
- Misses        749      772      +23     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.