code: @FromString
      public static Weeks parseWeeks(String periodStr) {
          if (periodStr == null) {
              return Weeks.ZERO;
          }
          Period p = PARSER.parsePeriod(periodStr);
          return Weeks.weeks(p.getWeeks());
      }
docs: Creates a new <code>Weeks</code> by parsing a string in the ISO8601 format 'PnW'.
  <p>
  The parse will accept the full ISO syntax of PnYnMnWnDTnHnMnS however only the
  weeks 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 weeks
  @throws IllegalArgumentException if the string format is invalid