New Coil-n-Currents, Derivatives, and stage II coil optimization

Courtesy @florianwechsung, this release introduces new Coil, Current, and Derivative classes and allows for stage-II optimization

• A new Coil class that represents a Curve and a Current. A list of Coil objects is now passed to BiotSavart (instead of separate curves and currents in the past). These objects are built on top of the new graph framework

• A new Derivative object is introduced, that is essentially a dictionary containing the partial derivative of a scalar objective wrt to all the Optimizable objects (with >0 DOFs) that it depends on.

• Simsopt can now solve the Stage II coil optimisation problem.