OPAN_XYZ: 'None' tweak completed for Dist_iter
OPAN_XYZ: 'None' tweak completed for Dist_iter

Code is likely best refactored to a helper function, into which
  can just be passed the arguments list of each xxx_iter function.
  Will simplify implementation in the other methods and also just
  be a lot cleaner.

bskinn/opan,bskinn/opan