ci(codecov): add codecov token

[d33bs]

Description

This PR adds a token for use with Codecov so that automated runs may complete.

Closes #441

What is the nature of your change?

• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• This change requires a documentation update.

Checklist

Please ensure that all boxes are checked before indicating that a pull request is ready for review.

• I have read the CONTRIBUTING.md guidelines.
• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• New and existing unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have deleted all non-relevant text in this pull request template.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.72%. Comparing base (c782728) to head (62fe3dc).
Report is 31 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #446      +/-   ##
==========================================
- Coverage   94.72%   94.72%   -0.01%     
==========================================
  Files          56       57       +1     
  Lines        3147     3145       -2     
==========================================
- Hits         2981     2979       -2     
  Misses        166      166              

Flag	Coverage Δ
unittests	94.72% <ø> (-0.01%)	⬇️

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[gwaybio]

LGTM once tests pass

[d33bs]

Thanks @gwaybio ! Merging this in.