Add support for playing a scenario at an individual speaker directly

[ewjmulder]

This is the same idea as fixed speaker ratios, but can be implemented even easier when taken into account explicitly.

[ewjmulder]

Convenience function on FixedVolumeRatiosAlgorithm to select 1 speaker for full volume and the rest volume 0. Only downside is that the room must be provided as well to map all other speaker id's. Future idea might be to generalize the speaker volumes to a function instead of a map (which can be backed by a map). But this makes less sense, cause mostly there is no logical value for a non existing speaker id and it should even fail then. So better leave it as it is.