Issue with `Atmosphere.rays` in 2D

[Goobley]

Seems that all 3 directions need to specified or a buffer error happens when the Context is being set up.

i.e. atmos.rays(muz=1.0, wmu=0.0) doesn't work, but atmos.rays(muz=[1.0], muy=[0.0], mux=[0.0], wmu=[1.0]) does.