fixed issue for take_one_step

This fixes an issue when using PyClaw with PeanoClaw. Since PeanoClaw always computes a single timestep, take_one_step is true in evolve_to_time. When the initial dt was too large, no timesteps where taken at all.

The fix leaves the functionality untouched when running evolve_to_time to take several timesteps (as usually done), but fixes the issue for taking a single timestep.

nosetest provides the same results as without the fix.

Regards,
Kristof