Improve 15 / 30

* [35bb43a](https://github.com/kangwonlee/nmisp/commit/35bb43ab3ac82f2dcb162a6c0a7f11bae3d1dbe5)
* [ ] Introduce hypothesis function $H(x_i)$
* [x] "Generating two data sets" : explicitly indicate $\overline x $
* [x] "Visualizing the two data sets" indicate $\overline x$ points
* [x] $\hat y = 0.5$ → decision boundary 04a8d44e
* [ ] "Can you expect any possible issues?" not explicit . Linear function range unbounded but $y \in \\{0, 1\\}$
* [ ] compare `scikit` result vs `scipy.optimize` result
* [ ] Logistic function video stop time : indicate play until the end
* [ ] Cost functions plotting intermediate steps
* [ ] See if we can visualize whether $-log()$ makes cost functions smooth