Phase out InverseRTransform and use forward transforms directly

[Ao-chuba]

closes #308.

• Deprecated InverseRTransform and preserved it purely for backward compatibility with a DeprecationWarning.
• Refactored solve_poisson_bvp and solve_poisson_ivp to accept forward transforms directly.
• Refactored solvers in ode.py to consistently operate on the finite integration domain using transform.inverse.
• Updated internal coordinate mapping to use BaseTransform.deriv_inverse and related methods instead of wrapping.
• Refactored BVP and IVP tests in test_ode.py to reflect the correct forward transform bounds and ensure strict monotonicity.
• All internal usages of InverseRTransform have been removed from the codebase.
• The only lines missing in the coverage are from InverseRTransform, which is deprecated and will be removed soon.

@marco-2023 Since InverseRTransform is not being used no more, would you prefer I leave it deprecated for a release cycle as currently implemented, or should I go ahead and completely remove the class and its unit tests right now in this PR?

do let me know if further changes required.

[Ao-chuba]

When call passes InverseRTransform to solve_poisson_bvp, the solver crashed inside _solve_poisson_bvp_atomgrid with: ValueError: The codomain of the transform (-1, 1) should be in [0, infinity).

This happened because InverseRTransform maps its own codomain to the underlying forward transform's domain (which has negative values, e.g. [-1, 1]), failing the non negativity check. To prevent this crash and maintain backward compatibility, we now automatically unpack InverseRTransform into its underlying forward transform at the entry points of solve_poisson_bvp and solve_poisson_ivp.

[Copilot]

Pull request overview

This PR continues the #308 migration away from InverseRTransform by deprecating it and updating Poisson/ODE solvers (and their tests) to operate using the forward transform plus BaseTransform’s inverse-derivative helpers.

Changes:

• Deprecates InverseRTransform (kept for backward compatibility via DeprecationWarning) and updates call sites/tests to pass the forward transform directly.
• Refactors solve_ode_ivp / solve_ode_bvp and internal ODE transformation helpers to solve on the finite domain using transform.inverse(...) plus deriv_inverse* methods.
• Updates Poisson solvers and ODE/Poisson tests to match the new forward-transform convention and revised bounds.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/grid/tests/test_poisson.py	Updates Poisson tests to pass forward transforms directly (no InverseRTransform).
src/grid/tests/test_ode.py	Refactors ODE tests for the new “solve on finite domain” transform convention and uses deriv_inverse*.
src/grid/rtransform.py	Adds a DeprecationWarning and deprecation docs to InverseRTransform.
src/grid/poisson.py	Updates Poisson solver docs/validation for forward transforms and adds backward-compat unwrap for InverseRTransform.
src/grid/ode.py	Refactors ODE solvers/helpers to use transform.inverse and inverse-derivative methods consistently.

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[Ao-chuba]

updated the pr,
cc @PaulWAyers @marco-2023
if any update required please do let me know