Ongoing default cleanup

DataSource Materials now all default to solid white.
Minor path visualizer cleanup.
Update CHANGES
Changes after review.

Better short circuting of branch logic in visualizers when an entity is not being displayed.
Clean up sensor API

This are changes needed ahead of #1887 to ensure API/CZML compatibility with 1.0
Undo unintentional variable rename.
Changes after review.

1. Make branch logic in PointVisualizer easier to follow.
2. Break out all DataSource defaults to module-scoped variables.
3. Minor tweak to CZML extrapolation processing.
Merge pull request #1929 from AnalyticalGraphicsInc/sampledProperty-extrapolation

Sampled property extrapolation and improved DataSource defaults
Default orientation for DataSource models

Eventually we plan on providing easy ways to specify orientation for Entity instances; but a small usability improvement in the short term is to just have the model stand up right if no orientation is provided.
Add camera doc, remove unused comment, fix 2D zoom and bounce animation.
Merge remote-tracking branch 'origin/master' into gltf-8
Merge pull request #1939 from AnalyticalGraphicsInc/camera-fixes

Camera fixes
Remove unused static toString functions.
Add toIso8601 function to TimeInterval to match fromIso8601.

Add toString functions on JulianDate and TimeInterval that use ISO8601 format.
Remove CZML backwards comapatibility code

We'll be standardizing on CZML 1.0 in the upcoming release and will only support 1.0 and newer going forward.
Merge pull request #1942 from AnalyticalGraphicsInc/toString

Date/Interval toString improvements for debugging
Combine TextureAtlas and TextureAtlasBuilder.
Fix specs and sample data after remove BC CZML code.
Update test data with regenerated czml files.
throw error of gltf text doesn't load
otherwise