use of doc strings in notebooks for visualization of test results

[petravanhoudt]

the notebooks to visualize the test results has background on test data and tolerances etc. These are now copy-paste (+edits) from the test files. This could probably be more efficient and automated by using doc strings in test files.

[petravanhoudt]

this could also help to add remarks to the notebooks, i.e. we have a 'notes' section in each notebook. We could use this to indicate where we needed modifications to the test data to get it run for each contribution. This might also be relevant information when you want to use the code.