#56: Improve cesium CZML support:
 - Support line strings and multi line strings. For multi line strings, the geometry is exploded into component line strings and each component line string is written out as a separate packet.
 - Extend the CZML example to have a point/line/polygon czml sources. This example is a bit heavy, we should look at simplifying it somehow.
 - Allow the CZML writer to gracefully handle:
    - Bad MgGeometry instances from AGF data
    - LineString instances with identical coordinates. Cesium doesn't like this, so such packets are not written.