Refactor test particles

[henry2004y]

This PR refactors the internals of the test particle module. More specifically,

• Add a new ParticleTrajectory class to combine the particle ID and particle data together.
• Refactor get_data with __getitem__, for more native Python operations.
• Add utility methods for obtaining vectors directly, get_kinetic_energy, get_pitch_angle, get_pitch_angle_from_v_b, and get_first_adiabatic_invariant. This can be further improved later.
• Add a more thorough type == "full" trajectory plot.

[codecov]

Codecov Report

Attention: Patch coverage is 42.16867% with 96 lines in your changes missing coverage. Please review.

Project coverage is 52.94%. Comparing base (ca49b0a) to head (ece81ea).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/flekspy/tp/test_particles.py	38.06%	96 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
- Coverage   55.56%   52.94%   -2.62%     
==========================================
  Files          12       12              
  Lines        1726     1800      +74     
==========================================
- Hits          959      953       -6     
- Misses        767      847      +80     

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