[WIP] ci: measure branch coverage

WHY IS THIS CHANGE NEEDED?:
- branch coverage give additional information re test coverage -- more
  informative for writing tests

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- add --branch flag, according to https://coverage.readthedocs.io/en/7.0.4/branch.html#how-to-measure-branch-coverage

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- will change the coverage report and the final percent coverage

REFERENCES:
- issue #46
- merge request !8