Prepare 1.0.0 tag (update CHANGELOG)

Nasrid exprequested to merge
prepare-release-tag into master

Prepare the creation of a new tag, 1.0.0, which will isolate the "old" version of this tooling. This is in order to have a clear split between this old version and the new upcoming version of the tooling, 2.0.0, that will work with RCMIP3.

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