Add codecov.yml to prevent failing pipeline on coverage decrease

[ashleve]

What does this PR do?

Previously any decrease in code coverage would fail the CI pipeline.

This seems bothersome for most projects, so this PR adds the codecov.yml file which allows to control the failure behavior and disables it by default.

Before submitting

• Did you make sure title is self-explanatory and the description concisely explains the PR?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you list all the breaking changes introduced by this pull request?
• Did you test your PR locally with pytest command?
• Did you run pre-commit hooks with pre-commit run -a command?

Did you have fun?

Make sure you had fun coding 🙃

[codecov-commenter]

Codecov Report

Base: 84.46% // Head: 84.46% // No change to project coverage 👍

Coverage data is based on head (91642c8) compared to base (75b44ff).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #484   +/-   ##
=======================================
  Coverage   84.46%   84.46%           
=======================================
  Files           9        9           
  Lines         354      354           
=======================================
  Hits          299      299           
  Misses         55       55           

Impacted Files	Coverage Δ
src/models/mnist_module.py	98.41% <0.00%> (ø)

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