Minor Python enhancements

[ChristopherRabotin]

Effects

• Rename DynamicTrajectory to TrajectoryLoader, since that's all it does
• Allow pickling of TrajectoryLoader
• Fix OEM parsing if multiple META sections
• Create StateParameter from its name (available in Rust but not in Python)

If this change adds or modifies a validation case

• No.

[codecov]

Codecov Report

Patch coverage: 86.66% and project coverage change: -0.07 ⚠️

Comparison is base (c9e51e4) 66.45% compared to head (13ef15c) 66.38%.

❗ Current head 13ef15c differs from pull request most recent head ccee18f. Consider uploading reports for the commit ccee18f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
- Coverage   66.45%   66.38%   -0.07%     
==========================================
  Files         127      127              
  Lines       32378    32391      +13     
==========================================
- Hits        21516    21502      -14     
- Misses      10862    10889      +27     

Impacted Files	Coverage Δ
src/io/trajectory_data.rs	0.00% <0.00%> (ø)
src/md/param.rs	58.97% <ø> (ø)
tests/propagation/trajectory.rs	0.00% <0.00%> (ø)
src/io/watermark.rs	96.55% <100.00%> (-0.51%)	⬇️
src/md/trajectory/orbit_traj.rs	92.15% <100.00%> (+0.11%)	⬆️

... and 8 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.