Atlas(Beta) function request

[BrandonGuo]

The new Atlas template is very impressive. I have some suggestions for improving its functionality:

- Enable the admin to configure the map's initial extent displayed when the Atlas app is opened.
- Enable the admin to choose which map is loaded by default when the Atlas app is opened.
- Allow to add layers from more than one group to the atlas.
- Enable the option to open scenes and maps in the atlas.

[AnnaHradecká]

Layers from more than one group is useful also for us.  

[IanWittenmyer]

Thanks for your suggestions.  The first two items have been implemented.  We are considering the other two for a future release.

[BrandonGuo]

@IanWittenmyer Please consider allow more than one groups to Atlas. It's really helpful if sharing Atlas with internal groups, while you want internal users also be able to add layers from Open Data Groups. 

Also, allow open scenes would be really helpful.