sampling notebooks require hotfixes

[bspanoghe]

• Basics notebook mentions "For-loops for many variables" section in intro notebook that no longer exists, needs to be added
• Naming convention of calling samples sp (for SteekProef) is outdated now that ProbMod pc practicals are gone, needs to change
• Intro notebook needs to show how to plot an analytical distribution
• Basics notebook requires warning that students are expected to add code cells in the probability notebooks
• Many students skip the blurred Hint in the basics notebook, change to Tip to remove blurring

[bspanoghe]

resolved by #72