OPAN_XYZ: Refactored 'None' substitution
OPAN_XYZ: Refactored 'None' substitution

Pulled the 'None' substitution and checking code into its own
  private method, and added option to Dist_iter either to error or
  to just report 'None' for invalid indices / combinations.

Still need to roll out to the other three generators.

bskinn/opan,bskinn/opan