Increase test coverage to 80%

[kdberends]

The current test coverage is 72.1%. Deltares quality goal is at least 80%. This issue is to push the code coverage to at least 80%.

At good place to start would be RegionPolygonFile.py, lib/polysimplify.py and Functions.py, which currently have the lowest test percentage. These modules are also candidates for refactoring:

• Functions.py are a relict from early function-oriented development. Most of these functions are called by only one specific class. See issue refactor Functions.py #98 on how to refactor (and test the refactoring!)
• RegionPolygonFile.py contains some code to do classification if the user does not provide pre-computed classification. There is a longstanding wish (Remove dependency on DeltaShell-modified netCDF files #25) to implement a different classification algorithm. Fixing that would mean a lot of code can be removed from this class.

Finally, in utils.py some edge cases are not tested.