ENH: displacementMotionSolver: additional hierarchy

- shm: have displacementMotionSolver as alternative mesh shrinker
  (instead of medialAxis).
- updated iglooWithFridges tutorial to use displacementLaplacian
- selectable interpolation from cells to points in the motion solvers
  using the 'interpolation' keyword:
    interpolation volPointInterpolation;    // default
    or
    interpolation patchCorrected (lowerWall upperWall);
- wrapped up mesh shrinkers (see above) for use as a displacementMotionSolver
  (i.e. the opposite of the displacementMotionSolver mesh shrinker)