ci: Allow reporting of coverage on PRs from forks

[matthewfeickert]

Description

Resolves #1559

On all events (with the exception on schedule events) triggered by maintainers on the main project upload coverage to Codecov using the secrets.CODECOV_TOKEN (which seems to be faster?). If the event comes from a fork, then rely on the fact that the token isn't required, and upload the Coverage results on pull_request events.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Run CI on push events to master by default, but on any branch on a push event if the commit message contains '[ci all]'
   - Results in a skip of push event tests on PRs by default
* Run CI on pull_request events to master
* Allow reporting of test coverage to Codecov on pull_request events from forks
   - Use fact that secrets.CODECOV_TOKEN is not required for public repos
   - c.f. https://github.com/codecov/codecov-action/blob/260aa3b4b2f265b8578bc0e721e33ebf8ff53313/README.md
   - On all events, except schedule, from maintainers, upload coverage to codecov.io faster using the CODECOV_TOKEN secret
   - On pull_request events from contributor forks, upload coverage to codecov.io without using CODECOV_TOKEN secret

[codecov]

Codecov Report

Merging #1622 (5cdaedd) into master (7ecaa63) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1622   +/-   ##
=======================================
  Coverage   97.70%   97.70%           
=======================================
  Files          63       63           
  Lines        4048     4048           
  Branches      576      576           
=======================================
  Hits         3955     3955           
  Misses         54       54           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.44% <ø> (ø)
unittests	97.48% <ø> (ø)

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[matthewfeickert]

Current result:

[matthewfeickert]

@kratsg I have the 'ci all' bit working now.

[matthewfeickert]

In the interest of moving other PRs forward I'm going to merge this in as we now have the [ci all] working in tests. If there are any further requests for edits I'm happy to implement them ASAP in a follow up PR.