new feature: particles with Stokes drag
feature/Stokes-drag contains an initial attempt to implement particles with Stokes drag. I had to extend the
particle_system classes a bit, and I had to create a new
particles_inner_computer_2nd_order_Stokes class, but I think I did it the right way.
@bbramas : when you have time, can you please have a look at my changes and confirm that they respect the philosophy behind the particle code?
@toba : if/when you have time, can you please design a sanity check for particles with Stokes drag, such that we can write a proper test for the new