Subregion type

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Use Case

VolumeCache and IVolumeLoader both define separate types to specify a volume subregion, with different functions to operate on them. As a developer, I'd prefer to work with a single unified type in these files and anywhere else we need to deal with subregions.

Solution

Create a single type for this purpose. Some thoughts:

• Consider using Three's native Box3 type.
• How should this type relate to LoadSpec? Should LoadSpec extend this type, convert to this type, or hold this type?

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Unit test operations on this type

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Resolved by #136 and #137