initial iterations in `incompressible` and `lm_atm`

[zingale]

These solvers call method_compute_timestep to get the intiial timestep for the initial iterations, but this doesn't have the safety checks that the main compute_timestep provides -- should they call the main version?

Also, since the pressure is lagged as p^{n-1/2}, is there a correction needed to account for the fact that dt is not the same between the previous step as it is for the next? (sources don't seem to deal with this)