Fix nested function calls in the forward mode

[efremale]

At the moment, calls in the forward mode work correctly only if called function has one input.
If a function f is called with args a1, a2, ..., its derivative should be a dot product of gradient(f)(a1, a2, ...) and a vector of derivatives of args: { da1, da2, ...}.

This dot product can be efficiently computed by differentiating f in the forward mode and providing values of da1, da2, ... as seeds. See issue #100.

[vaithak]

Similar to #100, this was fixed in #565 and many subsequent PRs which added the vector forward mode support.

We also have many test cases present with the vector mode interface: https://github.com/vgvassilev/clad/blob/master/test/ForwardMode/VectorMode.C.
Demo example: https://github.com/vgvassilev/clad/blob/master/demos/VectorForwardMode.cpp

[vgvassilev]

Thanks @vaithak, let's close the issue now.