Output contained in the .iter file:

Column 1: Radial profiles of density, pressure, and momentum from beginning of current time step
Column 2: Radial profiles of density, pressure, and momentum from beginning of previous time step
Column 3: RHS of transport matrix solve from beginning of current time step
Column 4: Estimated RMS error in profiles from last successfully completed iteration

These quantities are used if restarting in the middle of a time step