Automatically ignore Composites with all DoesNotExist() for finite differencing

[mzgubic]

Do we want this?

Closes JuliaDiff/FiniteDifferences.jl#149

[oxinabox]

I don't think we want this.
Special cases make it hander to maintain and use.
We have a type DoesNotExist which indicated to ignore it.
we should thus ensure when appropriate that type shows up.
we thus might want to normalize composites where all fields do not exist during the call to rand_tangent

[mzgubic]

Yeah fair. On the other hand I think @willtebbutt was reluctant to do a lot of special casing in rand_tangent

[oxinabox]

I think special casing in rand_tangent is better.
Is a set of simpler methods.

If we did automatic simplification in it for structs it generated that were all DoesNotExist fields to become DoesNotExist then that would cover same as this but be nicer