Function simultaneously

simultaneously(list_of_sounds: List): Sound
Makes a new Sound by combining the Sounds in a given list. In the result sound, the component sounds overlap such that they start at the beginning of the result sound. To achieve this, the amplitudes of the component sounds are added together and then divided by the length of the list.
Parameters
- list_of_sounds: List
  given list of Sounds
Returns Sound
the combined Sound
Example
```
simultaneously(list(sine_sound(200, 2), sine_sound(400, 3)))
```