OL-Cesium 1.33 release

[Guillaume Beraudo]

Hello,

I am pleased to announce version 1.33 of OL-Cesium, based on OpenLayers 4.6.2 and Cesium 1.40.

This release brings a lot of improvements:
  - Animate olcs.contrib.Manager#toggle3d.
  - Add support for Overlay synchronization, see example Overlays.
  - Port to OpenLayers 4.6.2 and Cesium 1.40.
  - Restore OpenLayers events propagation.
  - Workaround camera sinking under the terrain and finally jumping above it.
    See https://github.com/AnalyticalGraphicsInc/cesium/issues/5999. The
    workaround requires the Camptocamp version of Cesium (otherwise it has no effect).
  - Add preliminary work for an ES6 package https://www.npmjs.com/package/olcs.
  - Remove management of ol.source.ImageVector. This class is deprecated in OpenLayers 4.6.2.

As usual, see CHANGES.md.

Thank you to all contributors of OpenLayers, Cesium and OL-Cesium.

Especially, I would like to thanks Tim Schaub and Andreas Hocevar for the work they are doing in OpenLayers to make ES6 modules a reality.

Cheers,

Guillaume Beraudo