Add howto on attaching data to J_T

Author: goerz

docs/src/howto.md

@@ -24,6 +24,18 @@ lines = ["* [$(DocInventories.dispname(item))]($(DocInventories.uri(inventory, D
 Markdown.parse(join(lines, "\n"))
 ```
 
+## How to define a functional ``J_T`` that depends on more than just target states
+
+All of the optimization methods in the [JuliaQuantumControl organization](https://github.com/JuliaQuantumControl) target a final-time functional ``J_T`` via an argument `J_T`, see [`QuantumControl.optimize`](@ref), that has the interface
+
+> `J_T`: A function `J_T(Ψ, trajectories)` that evaluates the final time functional from a list `Ψ` of forward-propagated states and `problem.trajectories`. The function `J_T` may also take a keyword argument `tau`. If it does, a vector containing the complex overlaps of the target states (`target_state` property of each trajectory in `problem.trajectories`) with the propagated states will be passed to `J_T`.
+
+If the functional does not depend solely on a set of states `Ψ` and a set of corresponding target states, that raises the question of how to include additional information in `J_T`, within the constraints of the API that does not allow for additional arguments to `J_T`. For example, we might want to reference an operator in `J_T` whose expectation value should be maximized or minimized. There are three fundamental approaches:
+
+1. Hard-code the relevant data inside the definition of `J_T`. This is the most straightforward, but not very flexible.
+2. Attach the data to the function `J_T`. This makes sense when the data is independent of the `trajectories`. Typically, this is done via a [closure](https://en.wikipedia.org/wiki/Closure_(computer_programming)), e.g., via a `make_J_T(op)` function that returns a function `J_T(Ψ, trajectories)` that references `J_T`, or, in trivial cases, with an [anonymous function](@extref Julia :label:`man-anonymous-functions`) for the keyword argument `J_T` of [`optimize`](@ref). This is often the most flexible approach, but be aware of the [performance implications of closures](https://discourse.julialang.org/t/can-someone-explain-closures-to-me/105605).
+3. Attach the data to the `trajectories`. Each [`Trajectory`](@ref) takes arbitrary keyword arguments that can be used to attach any data as attributes that a custom `J_T` function may then reference. This makes sense when the data is unique to each `trajectory`. In fact, the standard (but optional!) `target_state` itself is an example of this; for functionals where a `target_state` does not make sense, it can be omitted and replaced by arbitrary other data, like maybe a `target_op`. Note that if that `target_op` is the same for all trajectories, it would be less redundant to associate it with `J_T`, see item 2.
+
 
 ## How to deal with long-running calculations