add postprocessing script to deduplicate cobertura reports

This finish adding Gitlab visualization of coverage reports, by deduplicating functions in the cobertura file to make it smaller than 10MB.

Not related to this MR, but related to coverage : can someone with access to repo settings go in Settings > CI/CD > General pipelines and set "Test coverage parsing" to ^Lines: ([\d.]+) %$, so Gitlab extract line coverage percents from build logs. It would allow Gitlab to fill the graph in repository analytics in generate badges like this