Implement test code coverage reporting

[aphalo]

This will help ensure that tests cover most of the code. The current approx. 5000 unit tests do not necessarily ensure full coverage.

[aphalo]

Attempts to get this working failed. Will try again later when I find time.

[aphalo]

Test coverage is still low at 49% and needs to be improved. Of course, reporting is a different issue, but computing covera with RStudio's add-on makes it possible to find what code is covered by tests and what code needs new unit tests.