Make all wrapped tools optional dependencies

[bjarketol]

Summary

• Move pywake, floris, foxes, wayve, and code_saturne from hard dependencies to optional extras (pip install wifa[pywake], wifa[all], etc.)
• Add wifa/_optional.py with a require() helper that raises clear ImportError messages with install instructions when a tool is missing
• Add pytest.importorskip() to each tool's test module so tests skip cleanly when the tool is not installed

Details

All API modules already used lazy imports (tool packages imported inside function bodies), so import wifa works without any tools installed. This PR formalizes that by:

1. pyproject.toml: Tools moved to [project.optional-dependencies] groups. scipy and pyyaml added as core deps (used at module level by API modules). mpmath moved to the wayve extra.
2. wifa/_optional.py (new): Central require(package, extra) utility using importlib.util.find_spec().
3. API modules: Each run_*() function calls require() at its top, giving actionable error messages before any tool import runs.
4. wifa/wayve_api.py: matplotlib and mpmath moved to lazy imports inside the functions that use them. FoxesWakeModel import moved into the foxes branch of wake_model_setup().
5. Test files: pytest.importorskip() added before tool imports so entire test modules skip when the tool is absent.
6. wifa/__init__.py: Added missing run_floris export.

Verified

• Bare install (pip install wifa): import wifa succeeds; calling any run_* gives a clear error with install instructions
• Single tool (pip install wifa[pywake]): pywake tests pass, other tools' tests skip
• All tools (pip install wifa[all]): all tests pass as before

Closes #50

Test plan

• pip install wifa (bare) — verify import wifa works, run_pywake() etc. raise clear ImportError
• pip install wifa[pywake] — verify pywake tests pass, other tests skip
• pip install wifa[all] — verify all tests pass
• CI pipeline passes

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