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
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
Start refactoring CZML to mandate versioning

This change breaks most CZML processing for now.  Also clean up some misc doc issues.
Ongoing CZML versioning work

1. Refactor how clock/name are processed.
2. Throw when the first packet is not a document object or is missing version.
3. Update specs.
update doc
Merge pull request #1945 from AnalyticalGraphicsInc/modelThrowOnLoadError

Model throw glTF file load error
Merge remote-tracking branch 'origin/master' into czml-cleanup
Merge branch 'czml-cleanup' into czml-versioning
fix typo
And CZML versioning tests and fix bugs uncovered by them.
Tweak CONTRIBUTING.md
Changes after review.
Fix CZML.
Merge pull request #1947 from AnalyticalGraphicsInc/czml-versioning

CZML versioning