added support for timestamps as well as years

temporal values are now compared as strings, e.g.

    ?obs <http://data.uba.de/led/temporal> ?time .
    ?time dct:start ?concept .
    FILTER(str(?concept) IN ("1980", "1980-01-01T00:00:00"))

this is required to support both xsd:dateTime and xsd:integer as
temporal values