Fix CZML cartographic handling

Remove CzmlCartographic and replace it with CzmlPosition, this causes Cartographic data to be converted to Cartesian on load, rather than every time it's needed.  This also fixed #636, which was the result of incorrect interpolation.  It also simplifies some code in DynamicPositionProperty.
Update CHANGES.md
Merge pull request #637 from AnalyticalGraphicsInc/issue636

Fix CZML cartographic handling