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)
Minor on writing style. I'd using as usual twice so close to each other. You can maybe think about another linker which comes to say the same.