should we tidy up the zone exclusion metadata?

[tomjnixon]

There are two issues:

• These use floats, when they should be doubles (matching the rest of the API, and less likely to suffer from loss of accuracy).
• Polar maxDistance and minDistance attributes are non-standard, and don't make sense anyway (as these select loudspeakers which always have a distance of 1).

Both are easy to fix, but would change the API; should we do this?