Create codecov.yml configuration file to relax approving PR

[joro75]

Provide a codecov configuration file that relaxes the codecov check that is required to approve the PR.
In the previous configuration a change of -0.01% of coverage blocked the pull request.
With this configuration a warning is only raised if the coverage is going 2% down. And because it is informational, it will still not block the actual PR.

Also see: https://docs.codecov.com/docs/common-recipe-list

[codecov]

Codecov Report

Merging #147 (b2e7844) into master (4dc80e5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #147   +/-   ##
=======================================
  Coverage   94.64%   94.64%           
=======================================
  Files          27       27           
  Lines        1325     1325           
=======================================
  Hits         1254     1254           
  Misses         71       71           

Flag	Coverage Δ
unittests	94.64% <ø> (ø)

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[DurgNomis-drol]

Good idea, is 2% enough?

[joro75]

I don't know if 2% is enough. It was blocking the other PR, so I needed a little bit.
Also be aware it is an 'informal' check now, so it will not be blocking any PR. So even with a 10% decrease we will still be allowed to accept the PR.