Add ray's impact parameter to light_ray_solution dictionary

[clairekope]

Having recently discovered where Trident stores ray metadata (see https://trident.readthedocs.io/en/latest/light_ray_generator.html#examining-lightray-solution-data), the impact parameter used to generate the ray should also be stored here if at all possible. Or if not in this existing datastructure, than as an HDF5 attribute or something. This will replace the need to store a separate impact_parameter.npy file.

[biboyd]

That makes sense to do and would be a lot cleaner. Having that separate file was always a janky workaround.