add clubb time-step to the atm.log

Clubb's time-step is not currently outputted anywhere. As clubb is probably the 2nd most expensive piece of the CAM code, it is important to keep track of its time-step as it relates to model performance.

At present, the clubb timestep, "dtime" is computed in clubb_intr.F90. It internally computes dtime from dt_macmic = dtphys/cld_macmic_num_steps. It solves for the smallest integer "nadv" one needs to divide dt_macmic by such that dtime <= clubb_timestep namelist var. So there is no guarantee that dtime = clubb_timestep namelist var.