[ff-pm-exercise] new naming conventions and remove deprecations warnings as possible

exercises/exercise-coupling-ff-pm/README.md

exercises/exercise-coupling-ff-pm/interface/CMakeLists.txt

...s/exercise-coupling-ff-pm/interface/freeflowsubproblem.hh

exercises/exercise-coupling-ff-pm/interface/main.cc

exercises/exercise-coupling-ff-pm/interface/params.input

...ercise-coupling-ff-pm/interface/porousmediumsubproblem.hh

exercises/exercise-coupling-ff-pm/models/CMakeLists.txt

...ises/exercise-coupling-ff-pm/models/freeflowsubproblem.hh

exercises/exercise-coupling-ff-pm/models/main.cc

exercises/exercise-coupling-ff-pm/models/params.input

.../exercise-coupling-ff-pm/models/porousmediumsubproblem.hh

exercises/exercise-coupling-ff-pm/turbulence/CMakeLists.txt

.../exercise-coupling-ff-pm/turbulence/freeflowsubproblem.hh

exercises/exercise-coupling-ff-pm/turbulence/main.cc

exercises/exercise-coupling-ff-pm/turbulence/params.input

...rcise-coupling-ff-pm/turbulence/porousmediumsubproblem.hh

...solution/exercise-coupling-ff-pm/interface/CMakeLists.txt

...n/exercise-coupling-ff-pm/interface/freeflowsubproblem.hh

exercises/solution/exercise-coupling-ff-pm/interface/main.cc

...ercise-coupling-ff-pm/interface/porousmediumsubproblem.hh