Add code and documentation for TrackPID

[lborrel]

Python code used to generate the TrackPID neural network model, along with documentation

[lborrel]

@AndrewEdmonds11 I realized recently that I completely forgot to upload my TrackPID code and documentation. Let me know if you have any comment

[oksuzian]

Took a look through this PR. A few things worth addressing before merging:

Bugs

• Broken README link. TrkPID/README.md links to [TrackPIDTrain.py](TrkPIDTrain.py) — target is TrkPIDTrain.py but the file is TrackPIDTrain.py. 404s on GitHub.
• NameError when switch_make_dataset=True. The main block calls make_dataset('mu', MU_dataset_name, "MU_temp.csv"), but only MU1_dataset_name and MU2_dataset_name are defined. The dataset-creation path is dead-on-arrival.
• apply_cut falls through silently on unknown particle. If particle is neither 'e' nor 'mu', mc_pdg / label_particle are unbound and you get an UnboundLocalError later. An explicit else: raise ValueError(...) would fail fast.

Nits / polish

• Magic constant 1695 in np.mod(array_mc['trksegsmc','time'], 1695) — presumably the microbunch period in ns; worth a named constant or comment.
• Docstring typo: "EvenNtuple" → "EventNtuple" in import_evtntuple.
• Outputs (PID_model.keras, train_history.json, TrackPID.onnx) are written to cwd. TrkQual/ uses a model/ subdirectory — might be worth matching that convention.
• README is missing the "table of commits per training version" that TrkQual/README.md establishes.