Add Coil Arclength Differential Variance Objective

[dpanici]

• Adds objective similar to the one detailed in Wechsung 2021 to penalize the variance of the arclength differential along a curve, which was claimed to be useful for avoiding issues with parameterization non-uniqueness in optimization.
• Also add jitable flag to compute to skip past checks, as in _CoilObjective we use coil.compute, and the transform check can run into issues with jit when the quantity requested has non-empty transforms (like "x_s" in this PR)
• [x] add a test for this
• [x] Add simplified version, this is equal to the simsopt implementation with nintervals="full"

From reference: