Inability to set contrast parameter for fleck.jax.ActiveStar

[taylorbell57]

At present, the fleck.jax.ActiveStar object cannot be passed in a spot contrast parameter and must instead use some combination of temperature or spectrum and T_eff or phot. This is not ideal, as in transit lightcurves the primary observable is the spot contrast, and the spot contrast is the parameter that must be used in the non-jax version of fleck. At first glance, it appeared that one could use ActiveStar.add_spot() to add spots with an input contrast value, but there are some bugs in that function where the contrast input isn't meaningfully used, and elsewhere in the code the spot contrast is always computed and no checks are done to see if the contrast even needs to be computed.

I strongly recommend you add support for contrast to be passed in through the __init__ function, that you make use of the contrast argument passed into the add_spot function (if not None), and within spot_coords first check if self.contrast has been set and then only compute contrast if it wasn't manually specified.

Let me know if you think it'll take you a while to get to this, in which case I might be able to submit a PR with my proposed changes to help expedite the process. I'm hoping to add support for jax and fleck.jax to Eureka! soon