Add code coverage to CI

[TTitcombe]

Description

Add codecoverage report to CI

Type of Test

• Unit test (e.g. checking a loop, method, or function is working as intended)
• Integration test (e.g. checking if a certain group or set of functionality is working as intended)
• Regression test (e.g. checking if by adding or removing a module of code allows other systems to continue to function as intended)
• Stress test (e.g. checking to see how well a system performs under various situations, including heavy usage)
• Performance test (e.g. checking to see how efficient a system is as performing the intended task)
• Other...

Expected Behavior

A code coverage step in the test script.
Each PR should generate a report on code coverage.
If coverage goes below 95%, the build should fail

Additional Context

See the PySyft test script

[pavlos-p]

@TTitcombe Should we leave this Closed for now, since it has been removed from #23 and re-implement it in the future?