Release 6.6.1

We are ready to release the new OTB version 6.6.1.

The following steps need to be performed :

1. Version number update in source code (branch release-6.6)
2. SuperBuild archive update (needed!)
3. Gather feedbacks on bugs that need to be fixed, make a public list and track the fixes to be able to get progress info during the release ( see #1722 (closed) for the bugs list )
4. Update release notes (walk the GitLab MR merged history and log all improvements) for OTB, Monteverdi and Ice.
5. Update the date in RELEASE_NOTES.txt
6. Sanity check the binary packages
   1. Windows
   2. Linux
   3. Mac
7. Tag the release branches
8. Merge the release into master (Not needed as this is a bugfix release only the release note should be back-ported)
9. Prepare and upload source packages
10. Promote nightly packages
11. Update documentation
    1. Software Guide
    2. Cookbook
    3. Doxygen
    4. Application online documentation
    5. WordPress page "Home" and "Download" pages
12. Upload OTB source archive to Zenodo to create a unique Digital Object Identifier (DOI)
13. Update OTB-Data-Examples.tgz on orfeo-toolbox (packages)
14. Send email to mailing list to announce the release
15. Release announcement on the blog
16. Announcement on social networks (twitter, google+)
17. Forward announcement to news_item@osgeo.org (OSGeo news)
18. Plan the next release (nominate new release manager, setup PSC meeting on IRC)
19. Contact QGis processing plugin maintainer to update XML description for new OTB-Applications (or supply it)