Aggregators Min and Max call Ordering file

[jitsedesmet]

This PR aims to resolve comunica/comunica#1212.
According to the spec, Min and Max should make use of the SPARQL ORDER BY ordering definition. I assume sparqlee implements this in Ordering.ts (Both the spec and the implementation are wild to me, so please correct me if this assumption is wrong).

The issue also makes me wonder whether there are problems regarding date (and similar types) within the other aggregators, since all aggregators call the typedValue of a literal. Before #163 this was always some basic JS type (if I recall correctly), but now there are also object typedValues, these do not implement the + operator for example. The other aggregators do call the regularFunctions so should work.

[coveralls]

coverage: 100.0%. remained the same when pulling de4cc48 on jitsedesmet:fix/min-of-date into 6c22f37 on comunica:master.

[jitsedesmet]

This seems to resolve the problem.
Query:

SELECT (MIN(?of) as ?min) (MAX(?of) as ?max)
where {
    ?p ?o ?of.
}

over:

<http://example.com/apple> <http://www.w3.org/2001/XMLSchema#datePred> "2010-06-21+08:00"^^<http://www.w3.org/2001/XMLSchema#date>.
<http://example.com/pear> <http://www.w3.org/2001/XMLSchema#datePred> "2010-06-21-08:00"^^<http://www.w3.org/2001/XMLSchema#date>.

gives:

[
{"max":"\"2010-06-21-08:00\"^^http://www.w3.org/2001/XMLSchema#date","min":"\"2010-06-21+08:00\"^^http://www.w3.org/2001/XMLSchema#date"}
]

[rubensworks]

Other than the single comment, this looks good to me!