scalar -> vector and vector -> scalar control flow

[ArnoStrouwen]

Currently derivatives of functions with scalar inputs and vector outputs are called gradients, and the control flow is mixed in with
vector -> scalar functions.
In my opinion, this is confusing and error-prone.

Additionally, for the former an in-place f! can make sense while for the latter it does not.

[ChrisRackauckas]

yeah that should probably just be Jacobian