- to conform in style with the rest, use underscore ee2ps_

- Do we need to have this persisted?
  I don't see a single use case in this pull request to use this association.
  If the use cases are limited (only for some debugging), move this out of here completely and store a corresponding ValueMap
(recall that we have a load of other SuperCluster collections where the PS association is completely irrelevant; even an empty ee2ps still gives 1% extra size)
  If the use cases will be fairly frequent (can't really ever see a 100%), move this out into an extra and only keep a Ptr or Ref to the corresponding product.