move symmetry handling logic into BaseGeom

Factor the common aspects of symmetry handling into BaseGeom. The only
difference between MeshGeom and LineGeom symmetry handling is how the
arrays are rendered. Everything else is shared between the two.
use bounding spheres for center and auto-zoom

That's a preparation for properly auto-zooming and centering objects
with symmetry-mates. Spheres are easier to handle, because they are
rotation invariant.
autoZoom is now based on the full assembly
appease grunt
fix rendering for assymetric units