Extend the documentation and error messages

[polsys]

• Closes Add a case study to documentation #28 by adding a case study with real-world data (meteorological measurements at Helsinki Kaisaniemi).
• Closes Add error message reference page #26 and contributes to Add more unit test cases #20 by improving the error messages for some corner cases. I decided that it is against the intended UX to have a page listing the possible errors.
• Includes some other small fixes to the documentation,