GC-14: Cleaned-up testing and development configuration and documentation:

- Enforce 100% coverage in tests (fail the tests otherwise).
- Updated development documentation to list:
- How to run tests with coverage.
- How to generate coverage in html format.
- How to build documentation.
- What tests are included when running test via tox.
- Default to running just the unit tests when not passing in any
arguments to pytest.
- Removed use of pytest-flake8.
- Do not run coverage tests by default outside of tox.
- Ensure documentation is built inside of tox environment, and that it
does not pollute the source directory.
- Have all Python interpreters specified in same section in tox
configuration file.