(Closes #445) upgrade to v4 of codecov action and supply token

[arporter]

[arporter]

I've added the CodeCov token to the list of 'secrets' for this repo. Upload to CodeCov was performed successfully (although CodeCov is still not happy because there's no coverage report for the base commit of this PR). Landing page now has a pretty picture :-) Ready for review from @sergisiso or @hiker.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.99%. Comparing base (a9ea1e9) to head (b98307d). Report is 15 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #446   +/-   ##
=======================================
  Coverage   91.99%   91.99%           
=======================================
  Files          85       85           
  Lines       13678    13678           
=======================================
  Hits        12583    12583           
  Misses       1095     1095           

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[arporter]

Thanks @sergisiso. I've googled and can find nothing to suggest we're doing anything wrong. Before this patch, uploads to CodeCov were failing with a 'missing token' error. This at least fixes that. I've looked at the CodeCov github app and that seems fine. The upload to CodeCov is fine. It just seems that CodeCov is not then reporting back to GitHub, possibly because it says it's missing coverage for the base commit. Therefore, I think this can go on as it is a small improvement over what we currently have.