BQL: Time anchor placeholders

[xllora]

There are some queries that only care about the oldest or latest incarnation of a triple base on the time anchor. Should BQL provide a place holder to allow early filtering? Currently this relies on sorting by time anchor and using limit.

[rogerlucena]

I believe this Issue may be closed given what was implemented for the FILTER keyword and the latest FILTER function with #149 and #150.