Implement temporal visualization. (2.3.0b)

Vizit can now visualize GeoJSON data temporally using LeafletSlider.
Since LeafletSlider incorporates with the spatial visualizer, some
cleanup will be required before release. Spatial visualizations are now
produced with all specified points in a single layer (radius indicators
are aded directly to the map). LeafletSlider requires a minor revision
in the interface (“timestamp” must now be “time”). Testing is still
required.