Set D8_Global 'time_step_type' attribute to 'fixed'
Set D8_Global 'time_step_type' attribute to 'fixed'

When set to 'adaptive', there doesn't appear to be a way to
stop the model. (This change should be reviewed by @peckhams.)

mdpiper/topoflow,mcflugen/topoflow,mdpiper/topoflow,mcflugen/topoflow