Dirk add current to client

Problem
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see RFC: https://github.com/nens/lizard-nxt/issues/322

Solution:
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We make a new ng directive called _temporal-vector-layer_, in which the logic for visualizing flow is implemented. Special care is taken to write an efficient implementation of selecting which datum to retrieve, given the current _timeState.at_ value. In a naive approach, (a lot of) unneccesary iterations through the series' timesteps would be performed. 

Also, we change the file app/lib/Layer.GeoJSONd3.js for the actual D3 calls: the code for **drawing** the SVG flow representations is to be found there.

x-referenced in lizard-nxt PR: https://github.com/nens/lizard-nxt/pull/333