10/10 `x_array` or `i in range()` ?

* [6ed408e](https://github.com/kangwonlee/nmisp/commit/6ed408ea521479370dbc2645352aafe02b90e544)
* sequential method do not have to store all available `x_i` values in the array
``` python
n = int((x_final - x_init)//x_interval)
x_i = x_init
for i in range(n):
x_i += x_interval
```
* should work & be memory efficient
* however not consistency with script cells and advantage of python `numpy` not as clear