Merge branch 'feature/exercise-fluidsystem-gnuplot' into 'master'

exercises/exercise-fluidsystem/2p2cproblem.hh

exercises/exercise-fluidsystem/2pproblem.hh

exercises/exercise-fluidsystem/README.md

...es/exercise-fluidsystem/components/plotdensityfunction.py

exercises/exercise-fluidsystem/exercise-fluidsystem_a.input

...cises/extradoc/exercise-fluidsystem_a_densityfunction.png

exercises/solution/exercise-fluidsystem/2pproblem.hh

exercises/solution/exercise-fluidsystem/CMakeLists.txt

...xercise-fluidsystem/exercise-fluidsystem_a_solution.input

...xercise-fluidsystem/exercise-fluidsystem_b_solution.input