Update answers
Update to answer 3b

extended irreducible error / Bayes error rate
improvements to 9d, 9f and 10d

- 9d: much shorter code
- 9f: correlation could between name and mpg if there were more data points n
- 10d: replaced range with histograms for visual inspection which gives more insight
Merge pull request #38 from rzijp/patch-2

elaboration on irreducible error