Add test coverage tools/metrics

**TL;DR**: Add test coverage metrics with [coverage.py](https://coverage.readthedocs.io/en/coverage-5.2.1/)

**Narrative**
As a developer, I want to have an idea of which code paths are well tested. This high-level overview will help us determine which code needs more thorough testing, and increase confidence in our releases as a result.

**Acceptance Criteria**
- [ ] [Coverage.py](https://coverage.readthedocs.io/en/coverage-5.2.1/) included in build/test process
- [ ] **Optional**: user [codecov.io](https://codecov.io/) (free since we're open source!)