Setup coverage upload to codecov

[BrianJKoopman]

Description

This PR does the initial setup of code coverage reporting to codecov.io. We just set this up on ocs as well (see simonsobs/ocs#411 and simonsobs/ocs#414.)

Motivation and Context

See simonsobs/ocs#411.

How Has This Been Tested?

This gets tested in the PR and on merge. Though I've tried to avoid the issues I ran into with ocs setup.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Repository infrastructure change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[codecov]

Welcome to Codecov 🎉

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 ☂️