Introduce Python and C++ code coverage reports and checks

[alejandrosame]

Description

This PR implements #256 and #257. The current minimum coverage requirement for the CI check is 80%, which is specified by the MIN_COVERAGE variable inside .env file. The Makefile has the following new actions:

• check-coverage-python: ensures that Python code coverage is at least MIN_COVERAGE%. Automatically called by make test.
• check-coverage-cpp: ensures that C++ code coverage is at least MIN_COVERAGE%. Automatically called by make test.
• show-coverage: collects coverage data generated during test execution and opens its report on a browser tab [see image below].
  

Affected Dependencies

Added the following python dependencies:

• coverage to generate Python coverage reports.
• gcovr to generate C++ coverage reports.

How has this been tested?

• make test executed succesfully
• CI changes tested in a test PR.

Checklist

• I have followed the Contribution Guidelines and Code of Conduct
• I have commented my code following the OpenMined Styleguide
• I have labeled this PR with the relevant Type labels
• My changes are covered by tests