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)
You can probably do another function for the plotting of the results in C.45, CHAID, and CART as well. In any case, since this code is much shorter, it may be somewhat an overkill -- you choose.