Added credits for DataSources and Model

[tfili]

• Added a credit options for CZML, KML and GeoJson data sources as well as for Model.
• Added ion credits to those classes as well that can't be removed from the instance.
• We now allow scene.postRender callbacks to add credits. This was required for DataSource credits.
• Added tests for all the classes.

[cesium-concierge]

Thanks for the pull request @tfili!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[tfili]

@lilleyse I just wanted to make sure what I did in Scene is ok. I don't see any ill effects from it.

[hpinkos]

I think hooking this in in Viewer is too high level. DataSourceDisplay requires a Scene, right? I would add a new beginFrame event to Scene that DataSourceDisplay listens to. This event would fire sometime between when creditDisplay.beginFrame and creditDisplay.endFrame are called in Scene.render. DataSourceDisplay would listen to that event and call the updateCredits function when that is raised. Does that make sense?

[tfili]

I think hooking this in in Viewer is too high level. DataSourceDisplay requires a Scene, right?

Agreed.

I would add a new beginFrame event to Scene that DataSourceDisplay listens to.

Is there a need for another event? There is already [begin|end]Update and a [pre|post]Render events. Is there any ill effects from moving them out to where I have them (moved out of local function render into Scene.render).

[hpinkos]

@tfili yeah, looking at the code, you'll either need to add another event, or move where creditDisplay.startFrame gets called.

[tfili]

This should be good. To test

• Start cesium server
• Run ion locally as a beta user
• Add a Model, CZML, GeoJSON and a KML and give them all attributions
• Click the View in Sandcastle link for each of them
• Verify you see your attributions

[hpinkos]

CHANGES.md?

[hpinkos]

Works great @tfili! Update CHANGES and I'll merge this in

[tfili]

@hpinkos Done!