Add unit test code coverage tool

[NorwegianKiwi]

Add https://github.com/istanbuljs/nyc as a tool to see how much of the code is covered by unit tests.

Note that it is NOT a goal to get 100% code coverage. Trivial functions, helper functions and functions that basically just use another function without adding any logic, are not necessary to write unit tests for. Therefore it is better to configure nyc so that it doesn't complain about functions we don't want to test.