hardcoded ldes element modification date path

[MikiDi]

LDES streams allows specifying the predicate path that needs to be used to get the last modified date of a (versioned) element. See the LDES spec regarding ldes:ldes:timestampPath. It would be nice if vocabsearch would leverage this metadata in order to fetch the modified date of an element, which is required to determine when to start a new unification session.
For now, since MarineRegions, nor BODC ldes streams implement this, we hardcoded the paths for known streams here.
Once some streams start implementing ldes:ldes:timestampPath, it would be nice if Vocabsearch made use of this. Some minor modifications to the architecture would be required, since ldes:ldes:timestampPath is specified at the Stream level, and the currently used ldes-consumer doesn't expose this data to the triplestore.