Radius edge quality metric for meshes

Implements the radius edge metric as a quality measurement for meshes (i.e. the ratio of the radius of the smallest sphere enclosing all nodes of a mesh element to the shortest edge of that element).
Basically this has all the advantages of the edge-ratio-metric and the equi-angle-skew-metric without any drawbacks that I am aware of.

I also use this PR to replace lots and lots of pointers in the metric classes with references.
Nice, did not know that I wrote it yesterday. :smile: