Add generation of level0Textures for HEALPixFITSLayer
   * HEALPixTiling starts from order 2 for now, no more level0Texture in GlobWeb
Adapt MocLayer to the new GlobWeb architecture
   * Moc starting order is equal to 2 for now
   * MocLayer became a post renderer
Remove loaded images on layer deletion
Decompress gzipped fits if needed
Some minor modifications
   * Uncheck fits on background layer change
   * Open error dialog if image url is unknown
   * Add accuracy parameter to histogram
   * Add barycenter computing to Utils
   * Fix selection bug if layer was deleted
Add SAMP extract button for UWS results
Add download button of features on visible tiles as VO
Use SiTools shortening url application plugin if defined