[slides] update coupling ff-pm slides
The following things have been changed
- columns are used to display pictures next to text
- adapt long equations to fit the slides better
- use Navier-Stokes equations instead of RANS equations (a lot more eqs would be needed for RANS and otherwise it would be very incomplete)
- introduce "supplementary material" slides that are there for self-studying (as time for presentation in course is too short, but information might be still interesting)