Evaluation of levelset on unstructured meshes

We may need to evaluate the levelset on unstructured meshes, for example for the evaluation of the distance function needed by some turbulence models.

Using the SkdTree it shouldn't be to hard to add explicit evaluation of the distance function on an unstructured mesh. It may be not super efficient, but it should do the job.

@haysam Do you foresee any issue?