Fixed the bug: events are now counted correctly

Problem
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The summary of events (in the extentAgg omnibox) uses a method for counting all currently displayed events (EventService.countCurrentEvents). However, this method counts all the seperate (D3) features, as oppossed to the events themselves. This is a problem because a feature can represent one or more events.

Solution
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Rewrite the method: countCurrentEvents to count events and not features.

NB
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Might fix issue (cannot reproduce the issue locally): https://github.com/nens/lizard-nxt/issues/261