code: public Related getRelated() {
  		String value = first(RELATED);
  		return (value == null) ? null : Related.get(value);
  	}
docs: <p>
  Gets the RELATED parameter value.
  </p>
  <p>
  This parameter is used by the {@link Trigger} property. It defines the
  date-time field that the property's duration (if specified) is relative
  to (for example, the start date or the end date).
  </p>
  @return the related field or null if not set
  @see <a href="http://tools.ietf.org/html/rfc5545#page-24">RFC 5545
  p.24</a>