Make accessing and mutating parent part of the API

[rofinn]

It'd be nice if there was a common API for accessing and mutating parent data for wrapper array types.

[ChrisRackauckas]

Yes, that is a very good idea.

iswrapped(A)
getparent(A)

?

[ChrisRackauckas]

should getparent also be applied to wrapped number types, like Dual numbers and TrackedReal?

[rofinn]

Yeah, I think that would make sense... assuming they're using the existing type hierarchy (e.g., TrackedReal <: Real).

[Tokazama]

We have parent_type now. You can do parent_type(T) <: T to confirm that T isn't a wrapper. Does that resolve this issue?