ENH: support unevaluated_expression default arguments

[redeboer]

Default arguments to the instance attributes of an @unevaluated_expresssion() class are now supported. In addition, usage of ClassVar is also better documented on the SymPy helper page.

For instance:

from __future__ import annotations

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

@unevaluated_expression
class FunkyPower(sp.Expr):
    x: Any
    m: int = 1
    default_return: ClassVar[sp.Expr | None] = None

    def evaluate(self) -> sp.Expr | None:
        if self.default_return is None:
            return self.x**self.m
        return self.default_return


x = sp.Symbol("x")
expr = FunkyPower(x)
assert expr.doit() == x

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