handled special case in anova_f_test, CHAID notebook added

- handled a special case in anova f test which was causing error in boston housing dataset
- CHAID section added in notebook

[View](https://gist.github.com/mazumdarparijat/985a516614cb358b8fb7)
I see this is the exactly same code you use to generate the data set in C4.5 and CART (as you say in the description). Please, create a function with this code and just call it from the three places, instead of copy-paste and have the same code in three places.

Sorry for being so annoying :-P