Refactor casadi conversion

[MarcBerliner]

Description

Refactors the casadi conversion functionality to live in the Symbol subclasses, removing the monolithic CasadiConverter

Also improves the build/eval time for the pchip interpolator:

Pchip build time (µs/call)

N	Before	After	Speedup
10	3.6 ms	311 µs	12x
100	33.5 ms	347 µs	97x
500	159.3 ms	439 µs	362x
1000	316.8 ms	566 µs	560x

Pchip evaluation time (batch=100, uniform grid, µs/eval)

N	Before	After	Speedup
10	54.9	11.2	5x
100	433.5	10.6	41x
500	2139.4	11.7	183x
1000	4400.7	12.4	355x

Fixes # (issue)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #)

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

❌ Patch coverage is 97.58713% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.36%. Comparing base (2de8f10) to head (68434cc).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pybamm/expression_tree/functions.py	90.47%	4 Missing ⚠️
src/pybamm/expression_tree/interpolant.py	97.05%	3 Missing ⚠️
src/pybamm/expression_tree/concatenations.py	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5436      +/-   ##
==========================================
+ Coverage   98.34%   98.36%   +0.01%     
==========================================
  Files         333      333              
  Lines       30480    30484       +4     
==========================================
+ Hits        29976    29985       +9     
+ Misses        504      499       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[BradyPlanden]

This is looking good, thanks!