Change document creation information back to a format compatible with SPDX

[goneall]

This is a structural change to the current model.

Currently, creation information is in the attributes author and specVersion.

In SPDX, these are in a single attribute creationInformation which contains this information. A previous version of the model used this approach.

[CASTResearchLabs]

in the current proposition, the creation information is handled by the author, created, populationMethod and specVersion attributes of the Document.
in case of multiple creations, isn't it a new document that was an edition of a previous document and that has new creation information?

[goneall]

isn't it a new document that was an edition of a previous document and that has new creation information?

Not really - we want to capture not only an individual that created the document, but also the organization and tool(s). One document with multiple creators (as opposed to multiple document versions).