ENH: add directionalMeshWave functionality

  For a given point within a given mesh, the existing `meshWave` method gives
  the orthogonal distance to a patch. In meshes with very steep terrain (e.g.
  a hill of 90 [deg], this might be problematic for the fields that require
  the distance to the patch associated with the terrain surface.

  `directionalMeshWave` is a variant of `meshWave` distance-to-patch method,
  which ignores the component in the specified direction. Can be used e.g. to
  calculate the distance in the z-direction only.

  TUT: add example of directionalMeshWave to mesh/moveDynamicMesh/SnakeCanyon

  Requirement by CENER
  Implementation by Mattijs Janssens