fix visualization output interval

This fixes the test composition-passive-tracers which happens to reach a
time step that is an exact multiple of the output interval. Bug was
introduced in 8e658c7b94f.
Merge pull request #239 from tjhei/fix_vis_output_interval

fix visualization output interval

This fixes the test composition-passive-tracers which happens to reach a
time step that is an exact multiple of the output interval. Bug was
introduced in 8e658c7.