Issues with definitions in the statistics chapter

[rikhuijzer]

Details at https://discourse.julialang.org/t/corrections-for-juliadatascience-book/69204.

• Remove the definition of a $P(A)$ as the "set"
• Fix Kolmogorov axioms
• Double-check the rest

[storopoli]

Remove the definition of a $P(A)$ as the "set" as noted by our friendly probabilistic comrade @cscherrer.

[colintbowers]

I left a comment on the Discourse thread, but just realised this issue was probably a better place for it. I just thought it worth pointing out that most mathematicians regard events of zero probability as possible, so I can see the sentence "The probability of an event is a real number between 0 and 1, where 0 indicates the impossibility of the event and 1 indicates the certainty of the event." ruffling a few feathers among the measure theory crowd. Here is a relevant stack exchange question.

[rikhuijzer]

Re opening because it is good to take a few more careful looks at it