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
ea16cb4b29