BSI ionisation correction for strong fields

[jtravs]

This pull request introduces support for barrier-suppression (over-the-barrier) corrections to the PPT ionisation rate.

Note this also changes the ionisation cache to be log spaced.

[Copilot]

Pull request overview

This PR adds barrier-suppression (over-the-barrier) correction support to the PPT ionisation rate, and updates the PPT acceleration cache grid to use log-spaced field samples (improving low-field resolution while allowing higher Emax without increasing N).

Changes:

• Implemented PPT barrier-suppression correction modes (bsi = :auto | :none | :tonglin | :zhang), including per-species coefficient lookup and enforcement that corrections apply only to tabulated atomic species.
• Updated PPT accelerated/cached rate generation to use a log-spaced field grid and bumped the cache version to avoid reuse of stale cache files.
• Added a dedicated BSI test suite and updated existing PPT regression tests/examples to pin bsi=:none where “bare PPT” comparisons are intended.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Ionisation.jl	Adds BSI correction implementation to IonRatePPT, introduces coefficient tables/dispatch (bsi_kwargs), and changes PPT cache sampling to a log-spaced field grid with a cache version bump.
src/Interface.jl	Documents the new default bsi=:auto behavior and how to override it via PPT_options.
test/test_ionisation.jl	Updates PPT regression checks to explicitly use bsi=:none to preserve “bare PPT” expected values.
test/test_ionisation_bsi.jl	New test coverage validating default behavior, suppression properties, known factors at E_b, and propagation through accelerated/cached PPT paths.
test/runtests.jl	Wires the new BSI test file into the test runner.
examples/low_level_interface/PPT_ionisation_rate/PPT_ionisation_rate.jl	Pins literature-comparison example calculations to bsi=:none so plots remain “bare PPT”.
examples/low_level_interface/PPT_ionisation_rate/bsi_correction.jl	New example script illustrating Tong–Lin vs Zhang correction factors and their effect on the PPT rate.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[jtravs]

Apparently these test failures are not related to this PR, but are just installing PyCall (the same tests worked fine last night). I will rerun them, but they shouldn't stop this PR.