new check - pubDate

[mobb]

pubDate is typed to xs:Date or xs:gYear
xs:Date is ISO, so, yyyy-mm-dd (confirm)
xs:gYear is pretty flexible. -- basically any integer (and negative too).

So knb-lter-nwt.735.2 has this:
<pubDate>20111231</pubDate>
It passes the EML parser, but it refers to a year many millennia in the future, not to a date (2011-12-31)

Check action: If content is only a single integer, it must be 4-digit (warn if not)

Because this is for pubDate, its safe to assume we will not be publishing data through the ECC in the year 10000

Checking format of another xs:Date might be more complex, so saving that for later