TST: Increase test coverage using pytest-cov, add documentation

[pllim]

It was not easy to run your tests using pytest-cov. When it did run, the coverage was only 29% (at the time of the original post of this issue) without CUDA enabled.

Would be nice if:

• Documentation on how someone would run this with pytest-cov is added and how to use it to measure coverage as intended.
• Developers to add tests to increase the existing code coverage.

Plus:

• Also run doctest.
• Use tox.