Detect common datetime and period patterns

[csarven]

SDMX data contains common datetime patterns e.g., xs:dateTime xs:date xs:gYearMonth xs:gYear, as well as periods e.g., xs:duration, or PeriodTypes in SDMX: YYYY followed with - and Q[1-4], W[1-52], T[1-3], B[1-2](See also: SDMXCommon.xsd). For such literals, use either URIs (e.g., reference year or quarter) or rdf:datatype with corresponding resource.

This is partially supported in issue #9.