datetime stamping should be possible, as date is not enough

[economidis-nick]

Documentation of file creation time by date (day-by-day) is not enough as a "timestamp". Companies export χMCF files more than once per day. Hence, a "timestamp" should include time within a day as well.

Considerations:

1. For compatibility reasons, we shall introduce a element in a way that existing files with element will not become invalid.
2. may only exist if exists.
3. must follow the ISO 8061: see wiki definition.

For benefit of international cooperation, users are encouraged to provide a time zone, but this is not mandatory.

[economidis-nick]

@DrCaFr please review the text added in document (section 7.1.2) and perhaps a couple of examples. I pushed it in a single commit, so that we can easily back it out if the committee does not accept it

[DrCaFr]

Solution reviewed and taken over to official ISO document.
Closing issue.