Reincorporate models of black hole mass and accretion rate

[aphearin]

In #111, we removed code implementing the Aird+18 black hole model, particularly the black_hole_accretion_rate.py and black_hole_mass.py modules. @yoquelbinsalcedo requested that we bring these ingredients back into the DESC mocks. So this issue can be resolved by a similar effort to what @evevkovacs is doing now in #112 for the B/T models.