Vector Adjoint Mode

[auto-differentiation-dev]

Allow rolling back multiple adjoints at once in the tape, e.g. for functions with multiple outputs.

The idea is to have an adjoint matrix instead of a vector. Multiple adjoints can be set at once, and then the library's computeAdjoints() rolls all of them back jointly. This would allow to calculate derivatives of functions with multiple outputs more efficiently, possibly using compiler vectorisation as well.