differentiate assignments of multiple values from functions

[David Tolpin]

It turns out to be handy to return multiple values from function. Try to find a way to differentiate such assignments.

[David Tolpin]

Instead of differentiating returns of multiple values, only methods returning a single float or nothing are differentiated. This allows to define helpers returning integers and other stuff on the model.

For clarity, a warning is printed if a method returning multiple floats is called in differentiated content. Neither the call nor the methods are differentiated.