Spin push for ExactSbend

[cemitch99]

This PR adds the spin push for the ExactSbend element. Consistent with the phase space push, which is analytically known in closed form, the spin map is analytically determined in closed form.

This is the most important and nontrivial example of an analytically-determined spin map with fully nonlinear dependence on the phase space variables.

• add draft of spin push, including evaluation of the first spin map factor
• add evaluation of the second spin map factor
• add benchmarks

Suggestions: consistency comparisons against Sbend and ExactCFbend with spin.
In particular, scaling tests can be performed (with respect to the phase space deviation and the map stepsize, respectively for the two comparisons above).

[codspeed-hq]

Merging this PR will not alter performance

✅ 39 untouched benchmarks
⏩ 2 skipped benchmarks¹

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_{Comparing cemitch99:add_spin_exact_sbend (7de8217) with development (472938d)}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩