ISO 8601 citation has been witdrawn

[Zarquan]

The citation for the ISO 8601 time and date format standard is no longer valid. We refer to a 2004 version, which has been withdrawn.

The citation itself is in the ivoatex module rather than in ExecutionBroker.

@Misc{std:iso8601,
  author={{International Organization for Standardization}},
  title={Data elements and interchange formats -- Information interchange -- Representation of dates and times},
  year=2004,
  url= {http://www.iso.org/iso/catalogue_detail?csnumber=40874}
}

We should update the ISO citation, but we should also cite the Wikipedia entry which is much more understandable.
https://en.wikipedia.org/wiki/ISO_8601

I must admit, I use the Wikipedia entry most of the time anyway.