Xpart

**Changes:**

- Move PDG ID functionality to Xtrack.

**Full Changelog**: https://github.com/xsuite/xpart/compare/v0.21.0...v0.22.0

**Changes:**
- Adapt `RFBucket` class ported from PyHEADTAIL to the Xsuite definition for RF Cavities.
- Support generation of matched bunched in the presence of reference energy changes.
- Replace argument `p_increment` (was in SI units of momentum) with `energy_ref_increment` (in eV) in bunch generation functions.

**Full Changelog**: https://github.com/xsuite/xpart/compare/v0.20.0...v0.21.0

**Changes:**
- Introduce `generate_parabolic_longitudinal_coordinates(...)`
- Introduce `generate_binomial_longitudinal_coordinates(...)`
- Introduce `generate_qgaussian_longitudinal_coordinates(...)`
- Introduce `generate_hypersphere_2D(...)`, `generate_hypersphere_4D(...)`, `generate_hypersphere_6D(...)`

**Full Changelog**: https://github.com/xsuite/xpart/compare/v0.19.6...v0.20.0

**Changes:**

- Add positron rest mass to `pdg.py`.

**Full Changelog**: https://github.com/xsuite/xpart/compare/v0.19.5...v0.19.6

**Changes:**

- Improve the robustness of tests by fixing some random number generator seeds.

**Full Changelog**: https://github.com/xsuite/xpart/compare/v0.19.4...v0.19.5

**Changes:**
- Apply fixes and relax some tolerances in tests after a new release of SciPy changes some of the constants in `scipy.constant`.

**Full Changelog**: https://github.com/xsuite/xpart/compare/v0.19.3...v0.19.4