Multiscale configuration API

[aliddell]

Changes

Added

• acquire-device-properties: A struct storage_properties_multiscale_s member multiscale to StorageProperties.
• acquire-device-properties: A corresponding entry in StoragePropertyMetadata.
• acquire-device-properties: A convenience function for setting multiscale properties, similar to the function for
  setting chunking properties.

Possible controversies

If anything is controversial, it'll probably be this one-member struct

struct storage_properties_multiscale_s {
    int16_t max_layer;
}

At one point there were other members for strategy and downsample factor. I'm expecting that these could come back at some point in the future.