Computing Jacobian for general limb-darkening model

[ericagol]

• [ ] Compute the Jacobian transformation with a least-squares fit to the non-linear coefficients integrated analytically over the range from \mu=0 to 1.
• [ ] Use this jacobian to transform the likelihood at the last step.
• [ ] Apply this to Claret's non-linear model and Maxted's power-2 model.
• [ ] See how this compares in precision and speed.

[ericagol]

We've described this in the paper, but leave it for future work.