Prior to this commit, vis-network worked well for our trackpad users and mousewheel users - zoom speed felt appropriate for both. Now, however, we're getting feedback that the speed is either too fast (from trackpad users) or too slow (from mousewheel users).

We could add a setting to our app for zoom speed, but we'd rather have the relationship between wheel speed and trackpad speed be fixed upstream. Two options appear to be:

1. Detect which one is being used for scrolling and then adjust the zoom by some fixed coefficient
2. Allow separate zoomSpeed settings for the two different scroll types.