ENH: mark @unevaluated arguments as non-sympy

[redeboer]

Major improvement to the @unevaluated decorator that makes it behave more like @dataclass. Arguments are now converted Field objects so that they can get properties. One of those implemented here is the 'sympified' specification (switched on by default). A new argument() field specifier (the equivalent of field() allows indicating that some arguments should not be sympified. This allows for constructs like the following:

>>> class Transformation:
...     def __call__(self, x: sp.Basic, y: sp.Basic) -> sp.Expr: ...
...
>>> @unevaluated
... class MyExpr(sp.Expr):
...     x: Any
...     y: Any
...     functor: Callable = argument(sympify=False)
...
...     def evaluate(self) -> sp.Expr:
...         return self.functor(self.x, self.y)
...
>>> expr = MyExpr(0, y=3.14, functor=Transformation)
>>> isinstance(expr.x, sp.Integer)
True
>>> isinstance(expr.y, sp.Float)
True
>>> expr.functor is Transformation

and is especially useful for defining classes like EnergyDependentWidth, which require a PhaseSpaceFactor protocol.