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

Add toString functions on JulianDate and TimeInterval that use ISO8601 format.
Merge pull request #1942 from AnalyticalGraphicsInc/toString

Date/Interval toString improvements for debugging
throw error of gltf text doesn't load
otherwise
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