Script "mathematical_optimisation_in_ml"

The script introduces some basic concepts of mathematical optimisation, explains gradient descent and also introduces the use of regularised optimisation in ML.

@chadhat @mikolajr @oschmanf

• I'm not sure if the full script fits into the available time of the workshop. I had some ideas how to explain the topic in my mind and wanted to write this down.

• The content could be placed before @mikolajr's introduction into classification methods and also would replace or simplify parts of @chadhat's deep learning scripts.

Feedback is very welcome!