Add Center Velocity as parameter for galaxy centering

[ufuk-cakir]

Currently, during galaxy centering we calculate the median velocity of the galaxy center

rubix/rubix/galaxy/alignment.py

Line 52 in 6c50c05

central_velocity = jnp.median(stellar_velocities[mask], axis=0)

But some simulation contain information about the velocity of the galaxy, for example in Illustris the SubhaloVel.

TODO

• load this field in the IllustrisAPI class: add this in the configuration yaml file
• make the center_galaxy function take an optional center_velocity parameter: If given, use it, else calculate median velocity