Clean up sensor API

This are changes needed ahead of #1887 to ensure API/CZML compatibility with 1.0.

1. Remove SensorVolumeCollection
2. Make `CZML` and `DataSource` interfaces consistent with latest spec
3. Replace `CustomSensorVolume` and `RectangularPyramidSensorVolume` `set/getDirections` with ES5 proeprty.  Also rename `material` to `lateralSurfaceMaterial` for API compatibility with other implementations.