Various improvements from RMS course feedback (and thoughts)

[mixmixmix]

• more numpy/scipy/pandas examples to practice
• more information on importing modules and practice with that (maybe how to check version, and example of errors when imported function doesn't exist. How to re-load and import after installing pip package? I think it is quite messy on the SO because there are so many ways to install packages...
• Adding "test your skill" tasks where we provide a task notebook/test which if solved correctly gives a good indication that a person mastered the material presented in the course (i.e. became a hero). This would be useful to help someone decide if their coding skills are sufficient to skip the course as a prerequisite
• Adding an information on "what's next" would be useful. I usually recommend hackerrank, but maybe there are other places where a person can improve their coding skills "by doing"?
• Having a cheat sheet for python packages we use. It is not in line with "search the docs", but this is the way how many people prefer coding. Turing my OpenCV years I kept most of the snippets I found on SO/docs in a txt file. One can make a case that it is important to learn order of variables in range() but I tried to with C++ for loop for many years and only when finally gave up and started using snippets, my work sped up.