Small tweaks to simulation loop for pseudo-realtime

I made the sleep time corrections in the main simulation loop a little more aggressive, and removed a redundant sleep command. On linux with boost, the timing accuracy was much tighter on a non-RT system, with 99% of delta-T falling within 1-3% of the desired actual timestep.  Obviously the accuracy depends on CPU load and other threads, and I'm not sure how windows' coarse timing will handle it, but for ubuntu these small changes are a nice improvement.