Path visualization improvements.

1. Added `DynamicPath.resoltuion` property for setting the maximum step size, in seconds, to take when sampling a position for path visualization.
2. Improved position sampling to take steps more evenly.  For example, previously, if data was at 61 seconds steps, and resolution was 60.0, steaps would be taken at 0, 60 and 61.  Now they will be 0, 30.5 amd 61.
Merge branch 'master' into pathResolution

Conflicts:
	CHANGES.md
Fixed BoundingSphere tests
Fix bad caching in DynamicPointVisualizer.

We were not only performing a bad color comparison, we were also caching a reference instead of cloning.  This resulted in point textures never being udpated to reflect the underlying DynamicPoint changes.

@shunter this is the issue I was just discussing with you.
Merge pull request #552 from AnalyticalGraphicsInc/fixPoints

Fix bad caching in DynamicPointVisualizer.
Cache DrawCommands in BillboardCollection.

Addressed issue #550

This seemed easy enough so I took a stab at it.  I kept it simple so we cache only as many as we need and discard them when we don't.
We could have a separte pool (or a generic ObjectPool) implementation if we want something more sphisticated, but I wasn't sure if the overhead was worth it.

What do you think @pjcozzi ?
Update CHANGES.md

Fix typo.
Merge pull request #547 from AnalyticalGraphicsInc/pathResolution

Path visualization improvements
Address issue #554

1. Make sure `Animation.destroy` unsubscribes from all events it subscribes to in the constructor.
2. `AnimationViewModel` should throw an exception if a `ClockViewModel` instance is not passed in.
Fix casing of "callback".
Merge pull request #556 from AnalyticalGraphicsInc/issue554

Address issue #554
Small JulianDate cleanups.

* Use a scratch variable for leap second lookups instead of allocating.
* Add an instance compareTo method, use that from LeapSecond.compareLeapSecondDate
Fix a number of places that were using reference equality on JulianDate instead of equals.
Fix typo.
Merge pull request #553 from AnalyticalGraphicsInc/billBoardDrawCommands

Cache DrawCommands in BillboardCollection.
Fix inconsistent name of cache variable
Merge pull request #558 from AnalyticalGraphicsInc/fixDateComparisons

Fix date comparisons to use equals
Merge branch 'master' into polylineMaterials
Fix offscreen rendering artifact.