MNT: Rename radius to radius_function in CylindricalTank and SphericalTank

[Copilot]

CylindricalTank and SphericalTank stored their scalar radius input privately (__input_radius) while the TankGeometry.radius property returns a PiecewiseFunction. This meant StochasticModel (which calls getattr(obj, input_name)) would get the Function instead of the original scalar when accessing obj.radius.

Changes

• CylindricalTank / SphericalTank: Renamed constructor parameter radius → radius_function; stored as self.radius_function (public), replacing the private __input_radius
• Backward compat: Old radius= keyword arg accepted via **kwargs with a DeprecationWarning pointing to radius_function
• Serialization: to_dict() now emits "radius_function" key; from_dict() handles both keys (deprecated "radius" triggers DeprecationWarning)
• Tests: Regression tests added for new attribute, deprecation paths (kwarg and deserialization), and to_dict key name

Migration

# Old (still works, raises DeprecationWarning)
tank = CylindricalTank(radius=0.1, height=2.0)
tank = SphericalTank(radius=0.05)

# New
tank = CylindricalTank(radius_function=0.1, height=2.0)
tank = SphericalTank(radius_function=0.05)

# Positional calls are unchanged
tank = CylindricalTank(0.1, 2.0)

After this change, obj.radius_function returns the original scalar input while obj.radius continues to return the PiecewiseFunction of height.

Pull request type

• Code maintenance (refactoring, formatting, tests)

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

Current behavior

CylindricalTank(radius=0.1, height=2.0).radius returns a PiecewiseFunction, not 0.1. StochasticModels that call getattr(obj, 'radius') to retrieve the nominal scalar value get the wrong type.

New behavior

obj.radius_function exposes the original scalar input. obj.radius is unchanged (still the PiecewiseFunction). Old radius= usage triggers DeprecationWarning.

Breaking change

• Yes

Keyword-argument callers using radius= will see a DeprecationWarning. Serialized data with a "radius" key in to_dict() output will trigger a DeprecationWarning on from_dict(). Positional callers are unaffected.