Store Anisotropy in Metadata

[william-silversmith]

This would be convenient for a number of applications, but wouldn't it make more sense for this codec to be minimal and be wrapped in e.g. h5 for metadata? On the other hand, anisotropy is applicable to a wide range of applications and describes the meaning of the voxels themselves and so might be important enough to justify inclusion.

The downside is that this would be a breaking change.