Feature: Implement NonlinearKicker element

[senyue-hub]

Description

This PR adds a new beam element NonlinearKicker for beam injection studies. It is based on the existing Wire element logic.

Key Features:

1. Vectorized Calculation: Allows simulating multiple wire sources in a single element

2. C Implementation: Core logic implemented in C with an optimized loop over wires, including singularity protection for numerical stability.

3. Validation: Verified against a reference sequence of standard xt.Wire elements in tests/test_nonlinearkicker.py.

4. Cross-Context: Compatibility (CPU/GPU) has been verified using xobjects test helpers.

Note on Physics Consistency: I noticed a potential discrepancy between the current Wire element code and the Physics Guide. It appears that the current implementation does not account for the wire tilt effect on the B field. I have kept the NonlinearKicker logic consistent with the existing Wire element for now to ensure a direct benchmark, but this might be worth investigating in a future update.

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (Verified with black)
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts