[ex3] implement executable solution

exercises/solution/CMakeLists.txt

exercises/solution/ex3/2p2cproblem.hh

exercises/solution/ex3/2pproblem.hh

exercises/solution/ex3/CMakeLists.txt

...tion/ex3/binarycoefficients/h2omycompressiblecomponent.hh

exercises/solution/ex3/components/mycompressiblecomponent.hh

...ises/solution/ex3/components/myincompressiblecomponent.hh

exercises/solution/ex3/components/plotdensityfunction.py

exercises/solution/ex3/exercise3.cc

exercises/solution/ex3/exercise3_a.input

exercises/solution/ex3/exercise3_b.input

...s/solution/ex3/fluidsystems/h2omycompressiblecomponent.hh

exercises/solution/ex3/spatialparams.hh