Recursively get property downwards for `Add` and `Mul` about sympy

[sylee957]

Not only Add don't have dual, but also most of the SymPy methods like Integer, Rational. I don't think that the discussion ends with Add and Mul, and the reason that .dual is not defined for most of the SymPy objects is that we need to discuss Add, Mul, Pow and in fact pretty much everything of Expr about how to handle .dual.

There would be easy or more obvious answers, such as defining def dual(self): self, however, I'm afraid that it doesn't work by simply working with Add, Mul exclusively and hiding the complexity about the discussion,

"send" a property or method call down the expression tree whenever it is invalid for Add or Mul.

It may be possible to override __getattr__ magic method, however, in general, I'd warn that such is very nonstandard programming practice, and we may encounter much more nonstandard compatibility or performance issues that not many people in Python community faces if we pick such direction.