Update AGN spin/jet model

This MR implements a variety of small changes related to the 'SPIN_JET' BH model. These changes are:

1. Some parameter updates as a result of calibration on 50 Mpc EAGLE low-res boxes.
2. Added the option of choosing any seed BH spin (this has also been tested - it seems to have little impact in cosmological boxes).
3. The floor spin value is now 0.01 instead of 0.001 (to prevent very small time-steps).
4. Added three new jet velocity models (and the necessary parameters). These use constant mass loading, or two different formulas for the velocity determined from local parameters (these derivations are on the COLIBRE-BH slack).
5. Small correction in the calculation of the jet velocity in the BlackHoleMass model; bp->group_mass was used instead of halo_mass, the latter of which is calculated a few lines above.
6. The minimal jet velocity parameter is now converted from km/s into internal units once read in.
7. Old AGN parameter AGN_cumulative_energy is now explicitly zeroed once BHs are seeded/read-in (for clarity).
8. Additional diagnostics - each BH now carries information on how much it accreted as well as how much energy it released as radiation/jets in each accretion mode (thick, thin and slim disc - these 3 modes are represented as parts of an array).
9. Further diagnostics - the total BH luminosity and jet power of all BHs are recorded in the statistics.txt file. This is non-trival to calculate (from the total accretion rate) in the spin model, where efficiencies vary from BH to BH.

Edit: It would be good to port these changes to the COLIBRE version if/once they are acceptable.