Setup coverage reporting on codecov.io

[BrianJKoopman]

Description

This PR sets up code coverage reporting on codecov.io. Before merging it might even remove coverage reporting to coveralls.

Motivation and Context

We've been using coveralls.io for a while now, but I often find the coverage reports are extremely slow to navigate, taking 10's of seconds or longer to expand directories/load source files. I'd like to migrate away to codecov.io for this reason alone, but codecov has some additional nice features that I find appealing. codecov is also used by several other repos in our organization.

How Has This Been Tested?

I've migrated sorunlib to codecov already, so this is based on that.

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.

[BrianJKoopman]

For reports to start working this needs to be merged. Replacement of coveralls will come in a follow-up PR.