add sleep time to show
In every effect there are at least one time the lines:
```python
lp.show()
time.sleep(t)
```
To write one less line every time after show, these lines could be:
```python
lp.show(t)
```
If lp.show() had a parameter for sleeptime and call time.sleep(t) if t > 0.
`t` has to be optional with default value `0`.
issue