Limb-darkening laws

linear_ld_law(mu, u1)	Linear limb-darkening law.
quadratic_ld_law(mu, u1, u2)	Quadratic limb-darkening law.
kipping_ld_law(mu, q1, q2)	Kipping limb-darkening law.
squareroot_ld_law(mu, u1, u2)	Square root limb-darkening law.
nonlinear_3param_ld_law(mu, u1, u2, u3)	Non-linear 3-parameter limb-darkening law.
nonlinear_4param_ld_law(mu, u1, u2, u3, u4)	Non-linear 4-parameter limb-darkening law.