fix issue where the timestep wasn't being limited in a system with only...

fix issue where the timestep wasn't being limited in a system with only massless remnants when fixed_timesteps should be triggered

fixed issue where the code got in an infinite loop after the above failure

fix TPAGB issue where massive cores were checked vs MCh just before the TPAGB: we now explicitly check that the star is a TPAGB star to do this