Include datatype by default in return value of Collection.get_relations()

[jesper-friis]

Currently has the rettype of Collection.get_relations() default value of the rettype="t", meaning that it only returns a (s,p,o) tuple. That makes it easy to miss the datatype for literals. Changing the default to rettype="T" will include the datatype and probably be safer.

The only issue is that it will create a backward incompatibility, so we should at least update the minor version number when doing it.