Refactor `ImageInfo`

[frasercl]

Use Case

ImageInfo is a legacy type whose fields are specific to a single data format and use a naming convention that we have to specifically ask the linter to ignore. As a developer, I'd prefer to work with a clearer, more general type to specify basic dimensions of a Volume.

Solution

• Move the legacy ImageInfo into JsonImageInfoLoader, where it is required to load data in this format.
• Write a new, more readable, more understandable type to use with all Volume objects.
• JsonImageInfoLoader should convert to this new type when loading.

Note

ImageInfo has been changed a bit by #130. It should be changed back when moving into the JSON loader to exactly match the JSON the JSON loader loads.