code: public BoxStoragePolicyAssignment.Info assign(String userID) {
          return BoxStoragePolicyAssignment.assign(this.getAPI(), this.getID(), userID);
      }
docs: Checks if there is already a Storage Policy Assignment and creates one if one does not exist.
  @param userID             the ID of the user you want to assign the Storage Policy to.
  @return information about this {@link BoxStoragePolicyAssignment}.