Check allowed ranges for xsd:YearMonth and xsd:YearMonthDay triples

[patrickbr]

This PR changes the behavior of the start_date and end_date writing behavior: if the xsd:YearMonth or xsd:YearMonthDay object would contain a month or a day outside the allowed range (1-12, 1-31, resp.), don't output it. It also adds some tests for this behavior.

I noticed that we output invalid values here while testing #58. During the E2E test, the TTL validation via QLever rightfully complains:

10:24:52 WARN  riot            :: [line: 9961932, col: 40] Lexical form '1831-18' not valid for datatype XSD gYearMonth