WIP: Keep track of original images

Please note that this pull request is based on #119 (PR #119 should be reviewed first).

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As discussed on the 18th of July, following #114, this pull request includes the following updates:
- Create a separate entity representing the original image file whenever an image is modified (e.g. `.img` -> `.nii.gz`) as part of NIDM-Results export.
- Move the `crypto:sha512` attribute from the `Map` entities to the image file entities.