[tutorial][ex3] add README and solution, finalize exercise

tutorial/CMakeLists.txt

tutorial/ex3/README.md

tutorial/ex3/components/mycompressiblecomponent.hh

tutorial/ex3/components/myincompressiblecomponent.hh

tutorial/ex3/ex3_a_problem.hh

tutorial/ex3/ex3_b_problem.hh

tutorial/ex3/fluidsystems/h2omycompressiblecomponent.hh

tutorial/ex3/spatialparams.hh

tutorial/extradoc/exercise3_a_solution.png

tutorial/extradoc/exercise3_a_solution2.png

tutorial/extradoc/exercise3_setup.png

tutorial/solution/ex3/h2omycompressiblecomponent.hh

tutorial/solution/ex3/mycompressiblecomponent.hh

tutorial/solution/ex3/myincompressiblecomponent.hh