def collection_site
      return unless medical_identifiers.size == 1
      site = medical_identifiers.first.site
      return if site.nil?
      site.site_type == Site::SiteType::COLLECTION ? site : nil
    end
def collection_site
      return unless medical_identifiers.size == 1
      site = medical_identifiers.first.site
      return if site.nil?
      site.site_type == Site::SiteType::COLLECTION ? site : nil
    end
Returns the collection site for which this participant has a MRN. If there is not exactly one
 such site, then this method returns nil. This method is a convenience for the common situation
 where a participant is enrolled at one site.

 @return [CaTissue::Site] the collection site