FILTER isTemporal

[rogerlucena]

This PR comes to add support for the isTemporal FILTER function in BadWolf, as detailed in #129. It also comes to help resolving what was requested by #115.

To illustrate its implementation on the driver side, this PR also adds support for this FILTER in the volatile OSS driver (in memory.go).

Now, the user can write queries like:

SELECT ?p, ?o
FROM ?test
WHERE {
    /u<peter> ?p ?o .
    FILTER isTemporal(?p)
};

That would return only rows for which the predicate ?p is temporal. For other examples of queries have a look at the newly added tests in planner_test.go.

This FILTER function also works for object bindings in the clause, along with predicate aliases and object aliases too.