if datatype is double, then format it as a double, even if it exists in
java as an integer

fixes test toRdf-0035
parse useNativeTypes option for fromRDF tests
Use Sesame NQuads parser if possible to compare blank nodes between documents

Some of the blank nodes are in the predicate position which is disallowed by RDF (excluding the unstandardised Generalised RDF), which makes it still difficult to compare

In addition, need to have a non-standard comparator to ensure that blank nodes in context position are not ignored in equality comparison