Add extension execises

[stevehadd]

For each of the notebooks, for each section of the notebook, add a cells describing things that participants can try to do on their own to test what have they have learnt in this section. For example suggest ways to imrpove performance or trying to use a different aglorithm within an existing pipeline. This can be done during the practical part of the tutorial if they have been following along executing the code during the presentation/theory part of the tutorial.
Based on suggestion from @owena11