Show code coverage for NGF PRs and Project

[mpstefan]

As a contributor of NGF
I want code coverage to be displayed as a badge on the repository and in every PR
So that I understand how my contribution affects code coverage and where the baseline is for the project.

Acceptance

• A badge which displays NGF code coverage is added to the README.
• The change in code coverage is added as a comment to every PR.

[mpstefan]

Discussed making this a bot comment on PRs for how much code coverage changed under certain circumstances.

[lucacome]

we can use https://about.codecov.io