Implement photodynamics

[ericagol]

• [ ] Allow integrator to compute positions and velocities (and derivatives) at pre-specified times.
• [x] Code up a differentiable light curve solver (such as Pàl's).
• [ ] Couple it to output of integrator by computing partial derivatives of light curve with respect to positions/velocities.

[langfzac]

Being done in a separate repo: Photodynamics.jl