Add pytest coverage

This branch adds pytest coverage tools to the master.

Multiple functionalities are added here

• Add instructions to view test coverage to the README
• Add the pytest-cov library to requirements-dev
• Add a yarn command to view test coverage.
• Optional: Check for output files resulting from pytest-cov and then use this to view test coverage in IDEs.
• Add coverage to the CI
• Add coverage to the README
• Add coverage XML report while also reading terminal ouput
• Add instructions on how to view code coverage in IDEs
  • Add instructions for Atom
  • Add instructions for PyCharm

Note: The coverage badge will display unknown since it is supposed to point to the coverage of master.

Closes #21 (closed).