Added code coverage report uploads to codecov.io

[gonzalocasas]

This adds code coverage reports of the core part of the library (excludes the cad binding packages) using codecov.io. This is how the report will start looking once we merge this into main:

It does not include coverage generated through doctest stuff, but we could add that as well. An open question is whether we'd want to create a compas-action for this, or if this is specific for the core only. I tend to think the latter for the time being, and we can move it to an action if this proves to be useful.

What type of change is this?

• [ ] Bug fix in a backwards-compatible manner.
• [ ] New feature in a backwards-compatible manner.
• [ ] Breaking change: bug fix or new feature that involve incompatible API changes.
• [x] Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [x] I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• [x] I ran all tests on my computer and it's all green (i.e. invoke test).
• [x] I ran lint on my computer and there are no errors (i.e. invoke lint).
• [ ] I added new functions/classes and made them available on a second-level import, e.g. compas.datastructures.Mesh.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added necessary documentation (if appropriate)

[codecov[bot]]

Welcome to Codecov :tada:

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered :open_umbrella: