Date Vector/Number to ISO 8601 Date String

Easy conversion of a datetime or a date vector or a serial date number into a date string. The date string may be an ISO 8601 timestamp or a single date/time value, selected by (optional) input token/s. Multiple tokens may be used to output multiple strings.

The ISO 8601 timestamp style options supported by this function are:
* Date in calendar, ordinal, 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.

By default the function uses the current time and returns the basic ISO 8601 calendar timestamp: this is very useful for naming files that sort alphabetically into chronological order.

### Examples ###

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

>> datestr8601()
ans = '19990103T150648'

>> datestr8601([],'yn_HM')
ans = '1999003_1506'

>> datestr8601(clock,'*ymdHMS')
ans = '1999-01-03T15:06:48'

>> [D1,D3] = datestr8601(now-2,'D','DDD')
D1 = '5'
D3 = 'Fri'

>> datestr8601(datetime,'DDDD','d*','mmmm','yyyy')
ans = 'Sunday 3rd January 1999'

>> [da,YWD,mmyy] = datestr8601([],'d*','*YWD','mmmm','yyyy');
>> sprintf('The %s of %s has the ISO week-date "%s".',da,mmyy,YWD)
ans = 'The 3rd of January 1999 has the ISO week-date "1998-W53-7".'

### 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'

### Single-Value Strings ###

* Consistent token syntax (UPPERCASE = week-numbering year, lowercase = calendar year).
* Easily adapted to other languages.
* Single-values include: day of the year or days remaining in year; year quarter (3-month or 13-week); year; month as digit or name; day of week as digit or name; date of month; hour; minute; second; deci/centi/milliseconds.