Replace coveralls with codecov

[BrianJKoopman]

Description

This PR should hopefully finish up the basic codecov setup, replacing coveralls completely.

Motivation and Context

See #411.

How Has This Been Tested?

Tested throughout #411, #412, #413.

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.