KML default altitude mode and line tessellate support

[schmidtk]

This fixes a couple issues identified in #907. Namely:

• I don't think we are properly defaulting altitudeMode to clampToGround
• I don't think we are properly reading tessellate, which should correspond to os.interpolate.Method.NONE if 0

The first is true of all geometries. The KML specification states altitude mode should default to clampToGround for all geometry types. The KML parser will now do this if altitudeMode is not set on the parsed geometry.

For the second item, if tessellate is disabled (0 or false) we will disable interpolation on the geometry. I also updated the Cesium converter to use Cesium.ArcType.NONE when interpolation is disabled. This will render geometries as defined by the KML spec.