Commit before merging
Merge remote-tracking branch 'origin/master'
New updating scheme for Brownian simulations.

Now, one can choose between three predictor-corrector procedures:
- one including only Brownian force, leaving 2 others with bare Euler
  method
- one including all forces, all of them evalated at time t, either
  in the prediction or in the correction step
- one including all forces, but using contact and hydro forces
  evaluated at time t for prediction step, and at time t+dt
  for correction step.

The last method should bridge the pure hydrodynamic simulation
with updating scheme correct at order dt^2 .