Acoustic Velocity Regions

[bmoos15]

I'm not sure how hard it would be but it would be nice if the couplant velocity would be applied to the region in front of the frontwall. Currently, the material velocity is applied to all. So I have to choose between the angle being accurate and the depth.

[MShields1986]

I started working on that here but never finished it or testing it, if you'd like to try and implement it in a separate branch in your fork I'd be happy to merge in a pull request, otherwise I'm not sure how quickly I'd get around to this.