FEAT: support class attributes in unevaluated_expression

[redeboer]

Warning

BlattWeisskopfSquared switched its arguments: z now comes before angular_momentum, so that angular_momentum can be used as a keyword argument, like in the example below.

The @unevaluated_expression() now supports class attributes by marking them with typing.ClassVar. For instance:

from __future__ import annotations

import sympy as sp
from ampform.sympy import unevaluated_expression
from typing import Any, ClassVar

@unevaluated_expression
class BlattWeisskopfSquared(sp.Expr):
    angular_momentum: Any
    z: Any
    max_angular_momentum: ClassVar[int | None] = None
    _latex = R"B_{{{angular_momentum}}}^2\left({z}\right)"

    def evaluate(self) -> sp.Expr: ...

z = sp.Symbol("z")
ff1 = BlattWeisskopfSquared(z, angular_momentum=1)
ff2 = BlattWeisskopfSquared(z, angular_momentum=2)
assert ff1.max_angular_momentum is None
assert ff2.max_angular_momentum is None
BlattWeisskopfSquared.max_angular_momentum = 3
assert ff1.max_angular_momentum is 3
assert ff2.max_angular_momentum is 3

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