BUG: spurious empty surface zones added (fixes #706)

- problems were introduced by the change ee252307 (issue #686).
  Affected reading of OBJ files.

  The fallback zone (used to catch unnamed groups/zones), which was
  previously filtered away when not needed. Now handle more explicitly.

ENH: use stringOps::split and low-level read{Label,Scalar} for parsing OBJ file