UnidataDD2MI.xsl changes for Date/DateTime

[brianmckenna]

We were having issues with non-dash/non-space date string:

2017
20171027
20171027T163259Z

all being output as <gco:Date>

This updates writeDate output for non-dash/non-space strings:

• length 4,8 are <gco:Date>
• length 16 is <gco:DateTime>

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