Add code coverage as a metric

[oswinso]

Currently, the only "metric" we have for our codebase is our CI tests for code compiling and the tests passing. Another metric I want to add is code coverage (even if it's not the best metric), to make the issue of our nodes not being tested more visible.

I am thinking of using something like codecov to add this metric.

Doing this will require the following steps (Guide for codecov here)

1. Add gcov to our builds, I think this should only require adding additional build flags to our builds if we're testing
2. Add command to circleci yaml file to upload coverage metrics to codecov
3. Add codecov coverage badge to README.md to flex over robocup's outdated coverage badge that hasn't been updated in over 4 years