directly reading BH accretion rate from simulation/caesar file as an option

[DhruvZ]

As an additional option, it might make sense to have powderday pull the black hole accretion rate directly from the simulation (for those that have black holes) rather than just having the user manually set it. This would make running many powderday runs with different accretion rates sourced from the simulation possible. This also is a value that could be pulled from a caesar file if supported is added there.