Gyr units is assumed when reading in BHmergers file (changa_bh_importer)

The output from ChaNGa is by default in simulation units for time, so a conversion is needed to compare with timestep times presented in Gyr. I think this is a holdover from the previous  .mergers file which was a post-processed object. Now merger information is output by default to a separate file. The format is correct, but the units are wrong.