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

WHY IS THIS CHANGE NEEDED?:
- pipeline failed: https://git.ecdf.ed.ac.uk/swain-lab/aliby/aliby/-/jobs/56303
  because ModuleNotFoundError: 'omero'

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- ci script had redundant lines: it run tests while ignoring network &
  pipeline, then ran tests again with coverage but without ignoring
  these
- ignored network because it requires 'omero'
- ignored pipeline because it requires specific files not accessible now

REFERENCES:
- issue #46

WHY IS THIS CHANGE NEEDED?:
- CI pipeline doesn't show code coverage of tests, therefore we can't
  evaluate how well the tests are at addressing potential issues

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- copying lines from
  https://docs.gitlab.com/ee/ci/testing/test_coverage_visualization.html#python-example
  for now, as i expect it to look something like that

REFERENCES:
- issue #46

[WIP] ci: visualise test coverage

WHY IS THIS CHANGE NEEDED?:
- evaluate & visualise test coverage, so that we can be better at
  writing code tests

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- chiefly taken from
  https://git.ecdf.ed.ac.uk/help/user/project/merge_requests/test_coverage_visualization.md#python-example
  but with some functions run under poetry for consistency

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- specific to GitLab 14.2.6 which we use.  coverage reports are handled
  differently in 15.0 and later versions: https://gitlab.com/gitlab-org/gitlab/-/blob/812408bafae328b81652110fc53c6c6b89e3db81/doc/ci/yaml/artifacts_reports.md#artifactsreportscoverage_report

EVIDENCE THAT COMMIT WORKS:
- minimal working environment: https://git.ecdf.ed.ac.uk/s1947236/code-coverage-sandbox/-/tree/3f11f55836a4605416508d146ae94b40af2720b8

REFERENCES:
- issue #46