Implement experimental PALS converter interfacing with the pals-python project

[cr-xu]

Adds PALS lattice conversion support for Cheetah interfacing with the pals-python project.

Essentially it builds a two-directional mapping between pals-python objects and Cheetah objects.

Description

• Adds cheetah.converters.pals with conversion to/from PALS objects and lattice files.
• Supports drift, marker, quadrupole, dipole, aperture, RF cavity, horizontal/vertical/combined correctors, and solenoid.
• Adds Segment convenience methods:
  • Segment.from_pals(...)
  • segment.to_pals(...)
  • Segment.from_pals_file(...)
  • segment.to_pals_file(...)

This is still an experimental implementation as the PALS and pals-python are both evolving.

To handle the extra fields:
PALS fields that Cheetah does not model directly are preserved in element.pals_extras.

• _extract_extras(...) stores unconsumed PALS fields/groups during PALS -> Cheetah conversion.
• _merge_group(...) and _split_pals_extras(...) re-emit preserved data during Cheetah -> PALS conversion, so that the round-trip is possible.
• Unknown PALS element kinds become cheetah.Marker with the full PALS payload stored in pals_extras, plus an UnknownElementWarning.
• Unsupported Cheetah elements or lossy conversions raise NotImplementedError.

Tests

Added PALS conversion tests for:

• In-memory round trips
• File round trips
• Extra PALS parameter preservation
• Unknown PALS element fallback
• Unsupported Cheetah element rejection
• New Segment PALS helper methods

Verified with:

• pytest tests/test_pals_conversion.py
• pytest tests/test_pals_conversion.py tests/test_lattice_json.py tests/test_bmad_conversion.py
• pytest --ignore=tests/test_3d_visualization.py

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code and checked that formatting passes (required).
• I have have fixed all issues found by flake8 (required).
• I have ensured that all pytest tests pass (required).
• I have run pytest on a machine with a CUDA GPU and made sure all tests pass (required).
• I have checked that the documentation builds (required).

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