Add shortcut for plurals to string.commify_with_units
Use string.commify_with_units plural shortcut
Tweak method order in mesh.cc
Add Model::surface method
Remove Model::bbox method

Callers can just use model->surface()->bbox() instead.
Remove SurfaceGroup::num_surfaces method
Add Surface::accum_stats method
Add Surface::stats method
snogray.lua: Print scene statistics