fig3_16.py

* [ ] `clf` -> `pl.clf()`
* [x] `[2 1]` -> `[2, 1]`
* [x] `[1 3 2]` -> `[1, 3, 2]`
* [ ] `0:0.1:6` -> `pl.arange(0, 0.1, 6+0.05)` may want to check branch [arange](https://github.com/kangwonlee/pyFPE6e/tree/feature/arange)
* [ ] `impulse(num,den,t)` -> [`scipy.signal.impulse((num, den), T=t)`](https://docs.scipy.org/doc/scipy/reference/generated/scipy.signal.impulse.html)
* [ ] `plot(t,y,'-')` -> `pl.plot(t,y,'-')`
* [ ] `grid` -> `pl.grid()`
* [ ] `title('Fig. 3.16 Example 3.23 system impulse response.')` -> `pl.title('Fig. 3.16 Example 3.23 system impulse response.')`
* [ ] `xlabel('Time (sec)')` -> `pl.xlabel('Time (sec)')`
* [ ] `ylabel('h(t)')` -> `pl.ylabel('h(t)')`
* [x] `% grid` -> `# grid`
* [ ] `nicegrid` -> need to implement a custom feature