Change volume based on distance between listener and source

[ewjmulder]

Currently, when calculating a dynamic volume, only the angle to the speaker is taken into account. But it makes sense that the same angle, but much difference in distance should also have it's effect on the volume. This might be tricky in the current setup, cause normalizing will remove this effect. Maybe it should be some kind of extra feature build in the system that the algorithms should be aware of. Needs some further thinking...
See also notes about this in #24.