refactor 'algorithm' definition in Checksum.json

[zopalmer14]

Updated the algorithm definition to remove references to ChecksumAlgorithm as it doesn't exist. Instead, it expects a string (simple solution). Will add a subsequent comment to PR that explains a potential alternative solution.

DOI DCAT-US 3.0 Documentation: Checksum - algorithm

Previous work: GSA/dcat-us3-tools@2a00049#diff-53ebd1692a2d4cfa5e4a21fd688a1e6846b4a546e02c47991bc42af76839d0d8

Should resolve: #22

[zopalmer14]

The aforementioned alternative solution (more complicated) that I propose for discussion is that the algorithm property instead checks for/expects a valid spdx:ChecksumAlgorithm value. To be more specific, it would validate that it receives an IRI that matches one of those within the 'has members' list, such as checksum algorithm sha256 (as seen in the DOI documentation example).

My worry with using this approach/solution is that it may be too prescriptive in that the list of checksum algorithms may not be exhaustive of all those used by agencies. Additionally, if there are new developments in the field, the list/controlled vocabulary may not be updated quickly enough to capture them.

[jbrown-xentity]

Looks good to me