Allow dual representation of 3D data

[jmacura]

Some data may come to HsCesium in a form, which has no direct counterpart in OpenLayers. Like CZML, glTF, 3dTiles, ...
There is a question how to represent these data in 2D mode = OpenLayers, what to display in the map and how not to loose the 3D information.
The idea is to create a "Layer" like usual, with some common "Source" for OL, but provide some custom property like "source3d" or "sourceCzml" etc. (or "czmlDataSource" to copy the Cesium native structure) in the layer's properties, so it can be processed by Cesium.