Ci coverage adjustments

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.
• bumpversion patch has been called on this branch
• The relevant author information has been added to .zenodo.json

What kind of change does this PR introduce?

• adds the doctests build to the needed builds for CI suite
• slight adjustment to the largely neglected makefile

Does this PR introduce a breaking change?

No

Other information:

Coveralls (https://coveralls.io/github/Ouranosinc/xclim) has been adjusted to be a little less discouraging, but a bit more strict (Coveralls will now be a required check). PRs now require the following:

• Coverage decreases can be no more than 0.25% from master
• Coverage total must remain above 80% (for individual runs)

Failing either of these conditions means the PR will be blocked from merging.

Additionally, Codefactor is now permanently ignoring "Complex Method" (Flake8: C901). This isn't great, but we're doing some relatively complex stuff, so let's be easy on ourselves.

[coveralls]

Coverage remained the same at 90.7% when pulling d73bd54 on ci_coverage_adjustments into d3d5db9 on master.

[Zeitsperre]

Settings don't seem to be working as expected. Cross-referencing the issue: lemurheavy/coveralls-public#1505