calculate model max radius from origin during load

Immediate usage is for Isometro Exporter (issue: isometro-exporter#30 (closed)) however this might also have some use for culling or something in more general usage eventually. Arguably calculating this where this MR calculates it is inefficient because it recalculates it for shared vertices, but for loading code that runs in the background, and being just a distance calculation, I think it's fine for the moment.