Set up codecov

[hunterhector]

Is your feature request related to a problem? Please describe.
codecov.io is enabled for this repository but the coverage runs on all individual projects that are not set up.

Describe the solution you'd like

1. Run coverage pytest on all modules, and make sure the coverage reports are correctly merged.
2. Simply run the command codecov to upload the reports. Check whether the coverage statistics are correct.
   • may need to set up codecov.yml to control the thresholds and ignores.
3. Make codecov as a merge condition.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.