Switch from coveralls.io to codecov.io

[john-science]

Reasoning

1. Recently, coveralls.io has broken a couple times and caused us build failures. On other projects, I have run codecov.io for years with no such problems.
2. Coveralls.io forces us to use a private GitHub security token. This is extra work we don't need to do with codecov.io. (Particularly since these tokens expire every 12 months, and we would need to keep people on top of that.)
3. Codecov.io will report any code coverage problems during a PR by opening a comment on the PR page. This is particularly handy, as developers can get a nice GUI explaining what lines of code they missed in their coverage.

In the end, this is a low priority issue, since what we have now is working. But, this is also an exceptionally low-effort issue to fix, as codecov.io is all but automatic.

[ntouran]

I have no problem with that.

[john-science]

There have been a couple of minor outages with coverals.io this week.

I thought I was going to close this ticket AGAIN, but now I'm thinking maybe it's still relevant.

[john-science]

Okay, in the past 13 months, coveralls has been pretty stable. So I will close this ticket, and only reopen it if we have more issues.

[john-science]

I thought I was past this. But coveralls.io broke again today:

https://github.com/terrapower/armi/actions/runs/3660420103/jobs/6188235179#step:8:773

report: commands[2]> coveralls --service=github
Not on TravisCI. You have to provide either repo_token in .coveralls.yml or set the COVERALLS_REPO_TOKEN env var.
Traceback (most recent call last):
  File "/home/runner/work/armi/armi/.tox/report/lib/python3.10/site-packages/coveralls/cli.py", line 64, in main
    coverallz = Coveralls(token_required,
  File "/home/runner/work/armi/armi/.tox/report/lib/python3.10/site-packages/coveralls/api.py", line 49, in __init__
    self.ensure_token()
  File "/home/runner/work/armi/armi/.tox/report/lib/python3.10/site-packages/coveralls/api.py", line 61, in ensure_token
    raise CoverallsException(
coveralls.exception.CoverallsException: Not on TravisCI. You have to provide either repo_token in .coveralls.yml or set the COVERALLS_REPO_TOKEN env var.

I have no idea why they are suddenly mentioning Travis CI. We have never used it.

I can't tell if our token expired, or they just changed their system, so we need a different token name now.

TBD.