Coils

[f0uriest]

Basics:

• each coil/curve has its own parameterization, knows how to compute basic stuff (position along coil etc)
• base class takes care of more general derived quantities
• make general compute method for coils with default grid/transform or user supplied (so we can use different grids for different objectives
• Objectives are bound to certain coils/coilsets/equilibria with build method
• Objective.compute uses bound coil methods to actually compute stuff
• Throw error if objectives not built before optimization - should throw deprecation warning now
• coils each have params method to get single vector of all their parameters
• also have method to parse their own params into sub-vectors (eg x -> Rn, Zn, current)
• move Objective.parse_args to Equilibrium as a way for it to unpack its own arguments
• Objective.parse_args should only split full x into pieces for each optimizeable object - equilibirum, coils etc take it from there
• All coils must be in a single overall Coilset (coilset can be made up of sub-coilsets)
• Coilset defines unique ordering for coils, allowing us to line up coils in state vector with constraints that apply to them.

TODO:

• [x] #513
• [x] #560
• [x] #561
• [x] #559
• [x] #496