Add new class for calculating wall distance

• Add geometry utilities for constructing spheres / bounding spheres / circumspheres (these are acutally not used by the wall distance but might be useful in the future now that they are implemented.)
• Add closestEntity and distance queries for point and AABB trees
• Add DistanceField a function-like wrapper around a set of geometries to make distance queries at a given sample point
• Add WallDistance a class that evaluates discrete distance-to-the-mesh-boundary field at either vertex or element centers

TODO

• better test
• Maybe use in existing RANS model
• fix check for UG or ALU in test
• check parallelism? maybe throw