code: @FromString
      public static Years parseYears(String periodStr) {
          if (periodStr == null) {
              return Years.ZERO;
          }
          Period p = PARSER.parsePeriod(periodStr);
          return Years.years(p.getYears());
      }
docs: Creates a new <code>Years</code> by parsing a string in the ISO8601 format 'PnY'.
  <p>
  The parse will accept the full ISO syntax of PnYnMnWnDTnHnMnS however only the
  years component may be non-zero. If any other component is non-zero, an exception
  will be thrown.
  
  @param periodStr  the period string, null returns zero
  @return the period in years
  @throws IllegalArgumentException if the string format is invalid