Summary of important user-visible changes for datatypes 1.3.0:
--------------------------------------------------------------

 ** The `datetime` class is complete.  Display and parsing run on a new
    LDML-subset `Format` engine supporting the full field set, the last
    query and conversion methods are implemented, and the two default
    display formats can be set per session with
    `datetime.setDefaultFormats`.  Parsing is 2500-3400x faster and
    rendering 22-310x, both having moved to compiled code.

 ** `datetime` supports leap seconds, through the `'UTCLeapSeconds'` time
    zone: the 60th second of a minute is a real instant on the 27 dates
    that have one, and elapsed time counts it.  Each conversion treats an
    inserted second the way its own format defines, as MATLAB does.

 ** Daylight-saving handling is corrected throughout.  A wall clock that
    its zone repeats or skips no longer aborts Octave; a repeated hour
    resolves to the later offset and a skipped one moves ahead by the
    length of the gap.  Each element carries its own UTC offset, so the
    two instants that share a wall clock stay distinct everywhere.  Zoned
    operations are also 4-40x faster.

 ** New binning: top-level `discretize` and `histcounts` functions, and
    methods of the same names on `datetime`, `duration` and `categorical`.
    Bin edges run down a ladder of calendar units, so a day bin starts at
    local midnight and is 23 or 25 hours long where a zone changes.

 ** `duration` counts milliseconds internally rather than days, removing
    the rounding that lost sub-second values, and gains `mod`, `rem`,
    `mldivide`, `mrdivide`, `eps`, `datenum` and `isregular`.

 ** Further new methods: `linspace` for `datetime`; `maxk` and `mink` for
    `datetime`, `duration` and `categorical`; `topkrows` for `datetime`
    and `duration`; `empty` for `datetime`, `duration`,
    `calendarDuration`, `categorical` and `string`; and
    `datetime.zoneNameStyle`, selecting how a zone name is displayed.

 ** `calendarDuration` is exact against MATLAB across its whole surface.
    `caldays` and `calweeks` refuse a span carrying months, directing the
    caller to `split`; `calweeks`, `calquarters` and `calyears` round
    towards negative infinity; arithmetic keeps the display units of both
    operands and refuses fractional scaling; and the display of times,
    infinities and negative spans is corrected.

 ** `size` with several output arguments follows core Octave and MATLAB
    for every class in the package: fewer outputs than the array has
    dimensions fold the trailing dimensions into the last, and more pad
    with ones.
