Prerelease v0.6

Use root CMakeLists.txt as the default version

Following on from !331 this patch fixes an issue where running
`scripts/run.py` without the `--ver` argument would result in the wrong
version number being generated in the spec HTML. The default value for
the `--ver` argument is now read from the `project()` command in root
`CMakeLists.txt` file, making it the single source of truth for the
version.