Triangular LUTs

[drobnyjt]

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Fixes #321

Description

This PR makes Za, Zb symmetric LUTs triangular - this gives a modest but measurable speedup

Tests

examples/test_rustbca.py

Todos

• Should add tests for Z_ion > Z_target, like neon on boron

[drobnyjt]

Some documentation on how much of a speedup all of this represents, for a near worst-case example (low-E, low N, low mass ratio, running via Python interface):

Even in this scenario, this PR offers a ~25% speedup over main. The benefit of the triangular lookup table is not apparent in this case, but it represents a small but measurable speedup for the standalone code of about a few % and a significant speedup for the smallest simulations.