LayerGroups basic raster sync

[klokan]

Cesium does not support concept of layer groups.

Basic raster synchronization as described in #10 iterates on in the list of OL3 layers - unsupported layers are ignored (LayerGroup is now one of them).

If LayerGroup should be supported we must include listeners on the childern of each LayerGroup in hierarchy for opacity, change of saturation, etc. and calculate the final opacity value of whole hierarchy of LayerGroups in the OLCesium wrapper.

[klokan]

A simple implementation which will include all layers in the LayerGroup hierarchy will be part #25.
The layers will be added individually and information about group is not passed to Cesium.

Advanced impementation of LayerGroups with listeners on opacity/hue/.. of the whole group is to be implemented later on.
Updated: there is #93 now.

[klokan]

The simple implementation done in f2709f2 and included in #25

[ahocevar]

Note from our client: "Our application uses several layer groups to make z-indexing and layer management very straightforward. The layer synchronization in ol3cs should be listening to change events from layer groups so layers stay synchronized between the map/globe."

[klokan]

Basic implementation for syncing layers which are inside of a LayerGroup has been done.
This is fully according the specification from #10.

Advanced implementation of LayerGroups - which would allow changes of opacity of a LayerGroup - influencing opacity on all individual layers inside of a group goes beyond the original specification.

@ahocevar - is there a bug report from the client on existing implementation of the LayerGroups?

[klokan]

Closing this ticket - if the advanced "Visibility & opacity of LayerGroups" should be implemented lets create a new ticket for that...
Updated: there is #93 now.

[schmidtk]

@klokan, the issue is with collection changes on layer groups, not visibility/opacity/etc. If a layer is added to/removed from the group that change should be reflected on the globe. We aren't changing display properties at the group level so we're not concerned about that.

[klokan]

OK. It is now more clear and it belongs to this ticket. Reopening.

[klokan]

Implementation available in the PR #88