Account for light travel time effects

[semaphoreP]

Is your feature request related to a problem? Please describe.
Account for the fact that when we receive photons from the planet and star, they were emitted at different times (and thus at different positions). This effect is biggest for edge-on orbits, systems closer to us, and for companions at wide separations. Here is how much this effect is for an edge-on circular orbit at maximum z-separation situated at 10 pc. It matters at GRAVITY precision starting ~30-100 au.

Describe the solution you'd like
Account for the fact the planet's light is emitted at a slightly different time than the star. This would involve needing to compute the planet's position at a slightly different time than the time received. This is negligible in almost all cases, so it should be optional.

Describe alternatives you've considered
We could also just inflate the errorbars to hide this effect.

Additional context
H/t to @sefffal for bringing it up.

[semaphoreP]

Would be interesting to check if integration time matters also. Also BJD/HJD differences?