Rename Objective to Trajectory

[goerz]

See JuliaQuantumControl/QuantumControl.jl#59:

The term "objective" was originally taken from the krotov Python package, but has been a considerable source of misunderstanding. People (rightfully) get confused about "objectives" which are separate from the optimization functional. Especially when talking about a perfect entangler optimization where the objectives/trajectories have no target state, I always have to explain that 'objectives' is a misnomer, and that these are just the states whose dynamics enter the functional. The term "trajectory" is more accurate, as it consists of the initial state and the dynamical generator. Any (optional) additional properties (target_state, weight, …) are best understood as extra information specific to that trajectory that the functional may use.

The release of QuTiP 5 seems like a good opportunity to release krotov v2.0, which in turn is an opportunity to make this breaking change. This will also bring the krotov package closer again to the newer QuantumControl.jl, and would have made the Python and Julia versions of the Tutorial for the recent Optimal Control Workshop more similar.