Improve test coverage

[crisluengo]

We need to add more tests for some stuff that was implemented before the test framework was integrated. Most of the library infrastructure is covered by tests, but not all.

Also many of the more complex image analysis routines do not have tests. They were tested when written, but it is not trivial to write an automated, thorough test for such algorithms.

~~Finally, we need to set up some form of continuous integration on GitHub, so these tests are run automatically for pull requests.~~

[slokhorst]

I will have a look at CI testing. #30 and #31 could have been easily caught if we would have this.