Fix abstract validation error and SpdxOrganization URI validation errors

[goneall]

I got the validation for the spdx-spec example to work fully with some changes to the spec parser plus the change to the model in spdx/spdx-3-model#937

I thought I would share the changes to the spec-parser as a pull request - feel free to re-implement.

This pull request fixes 2 issues in 2 commits.

The first commit implements the abstract class validation as suggested by @JPEWdev - #165 (comment)

This fixes #165

The second commit changes the URI for createdBys in the individuals from <https://spdx.org/> to ns1:SpdxOrganization

This fixes #166

Note that the order of the constraints are not the same as suggested - here is an example of what is output:

 sh:property [ sh:message "https://spdx.org/rdf/3.0.1/terms/Core/ElementCollection is an abstract class and should not be instantiated directly. Instantiate a subclass instead."@en ;
            sh:not [ sh:hasValue ns1:ElementCollection ] ;
            sh:path rdf:type ],

[zvr]

Merging this to allow validation.
Although in the future some of the magic info should be moved to model repo and not in the RDF generator.