Relax tolerance in fixed output times / fixed timestepping

Due to numerical issues for run times in the range until one second the threshold have to be adjusted. This issue was discovered by @cbsilver

1. Feature description was added to the changelog
2. Tests covering your feature were added? - Original issue from @cbsilver was adjusted to SteadyStateDiffusion process
3. Any new feature or behaviour change was documented? - no new feature was implemented