feat: separate velocity autocorrelation function

[jung235]

Add (unnormalized) $\mathrm{VACF}(t)$ to Trajectory and Ensemble.

[codecov]

Codecov Report

Merging #8 (57f9f21) into main (22e496a) will increase coverage by 10.51%.
Report is 12 commits behind head on main.
The diff coverage is 51.72%.

@@             Coverage Diff             @@
##             main       #8       +/-   ##
===========================================
+ Coverage   46.28%   56.79%   +10.51%     
===========================================
  Files          30       32        +2     
  Lines         955     1030       +75     
===========================================
+ Hits          442      585      +143     
+ Misses        513      445       -68     

Files	Coverage Δ
pydiffuser/_version.py	100.00% <100.00%> (ø)
pydiffuser/mech/fields.py	84.37% <ø> (ø)
tests/models/test_bm.py	100.00% <100.00%> (ø)
tests/tracer/test_ensemble.py	100.00% <100.00%> (ø)
tests/utils/test_jitted.py	100.00% <100.00%> (ø)
pydiffuser/tracer/component.py	73.68% <50.00%> (+5.26%)	⬆️
pydiffuser/_cli/cli.py	0.00% <0.00%> (ø)
pydiffuser/_cli/cli_utils.py	0.00% <0.00%> (ø)
pydiffuser/tracer/ensemble.py	42.85% <21.05%> (+8.62%)	⬆️
pydiffuser/tracer/trajectory.py	43.92% <21.05%> (+4.15%)	⬆️

... and 5 files with indirect coverage changes