ISO 8601 Date String to Serial Date Number

Easy conversion of an ISO 8601 formatted date string (timestamp) into a serial date number. ISO 8601 formatted timestamp/s within the date string are automatically detected, or by supplying an optional input token the timestamp/s format can be restricted to detect only one particular style.

The ISO 8601 timestamp style options supported by this function are:
* Date in ordinal, calendar, or week-numbering notation.
* Basic or extended format.
* Choice of date-time separator character.
* Full or lower precision (fewer trailing date/time units).
* Decimal fraction of the trailing unit (decimal places).

Does NOT parse or support timezones.

The function returns:
1) The serial date numbers converted from the timestamps in the input string.
2) The input string parts that are split by the detected timestamps.
3) A Cell Array of tokens giving the detected timestamps' styles.

### Examples ###

* Using the date+time given by date vector [1999,1,3,15,6,48.0568].

>> datenum8601('1999-01-03 15:06:48.0568') % calendar, extended
ans = 730123.62972287962

>> datenum8601('1999003T150648.0568') % ordinal, basic
ans = 730123.62972287962

>> datenum8601('1998W537_150648.0568') % week numbering, basic
ans = 730123.62972287962

>> [dtn,spl,tkc] = datenum8601('A19990103B1999-003C1998-W53-7D')
dtn = [730123;730123;730123]
spl = {'A','B','C','D'}
tkc = {'ymd';'*yn';'*YWD'}

>> [dtn,spl] = datenum8601('1999-003T15')
dtn = 730123.6250
spl = {'',''}
tkc = {'*ynTH'}

>> [dtn,spl] = datenum8601('1999-01-03T15','*ymd') % specify match token.
dtn = 730123.0000
spl = {'','T15'}

>> datevec(datenum8601('19990103 200012')) % default allows 'T @_' separator.
ans =
1999 1 3 20 0 12
>> datevec(datenum8601('19990103 200012','_')) % date-time separator char.
ans =
1999 1 3 0 0 0
2000 12 1 0 0 0

### ISO 8601 Date Notations ###

Timestamps are shown here in extended format with the default date-time separator character 'T'.

1) Calendar:
<year>-<month>-<dayofmonth>T<hour>:<minute>:<second>
string: '1999-01-03T15:06:48'
token: '*ymdHMS'

2) Ordinal:
<year>-<dayofyear>T<hour>:<minute>:<second>
string: '1999-003T15:06:48'
token: '*ynHMS'

3) Week-numbering:
<year>-W<weeknumber>-<dayofweek>T<hour>:<minute>:<second>
string: '1998-W53-7T15:06:48'
token: '*YWDHMS'