Implement date time arithmetic

[jindrichmynarz]

It would be useful to have date time arithmetic implemented in Halyard. SPARQL 1.1 supports it only for numeric data types, see XPath arithmetic. More details can be found in w3c-cg/sparql-dev#32 and w3c-cg/sparql-dev#55. The implementation can use the tests here.

[pulquero]

See https://github.com/eclipse/rdf4j-storage/blob/master/evaluation/src/main/java/org/eclipse/rdf4j/query/algebra/evaluation/impl/ExtendedEvaluationStrategy.java but note eclipse-rdf4j/rdf4j#1033.