Minimal bounding spheres for points

This introduces a method to create smallest enclosing spheres for an arbitrary number of points.
The implementation is a bit tricky due to the recursive nature of the algorithm but according to the papers it has a near linear runtime even for high-dimensional sets of points.
I will need this functionality for a mesh element quality metric (radius to shortest edge ratio) that is especially useful for the tetrahedral meshes we can now create. (You may know this from ParaView.)

I referenced the papers this is based on in the source file, if anyone is interested I can provide you either with PDFs or the printouts.
I just discussed this with Dima and we think it might make more sense to handle the mesh nodes in the metric class. The main issue is, that you would still have to handle different containers as mesh nodes are stored in array and not in vectors.