code: function( dt ){
  
              if ( dt ){
  
                  this._dt = +dt.toPrecision(4); // only keep 4 decimal places of precision otherwise we get rounding errors
                  // calculate the maximum jump in time over which to do iterations
                  this._maxJump = dt * this.options.maxIPF;
  
                  return this;
              }
  
              return this._dt;
          }
docs: chainable
  Physics.world#timestep( [dt] ) -> Number|this
  - dt (Number): The time step for the world
  + (Number): The currently set time step if `dt` not specified
  + (this): for chaining if `dt` specified
  
  Get or Set the timestep