Useful Tutorial/Examples

[dpanici]

Listing here some ideas for useful things to have examples for, anyone feel free to add

• [ ] How to access/use automatic differentiation derivative information (say, if you loaded in a DESC eq to your other code, how can you access useful derivatives to avoid doing finite differences thru equilibria?) - would need to use JAX anyways -> related to #861
• [x] How to write an Objective Function : explain build method more, what passing derivs= to a transform means, why we need to copy iota and pressure when an objective function depends on those two quantities (so if we modify the grid we don't modify the existign grid, which may be linear while we usually use ConcentricGrid for the collocation method)
• [x] small example showing how to check the objective compute value (objective.compute(*objective.xs(eq)))
• [ ] Update dev_guide as current has since been added as well as an argument
• [x] explain how fixed current mode and fixed iota mode differ