the documentNamespace argument should be checked for compliance

[c-dore-philips]

SPDX is very precise about the syntax of the docNamespace field
https://spdx.github.io/spdx-spec/v2.3/document-creation-information/#651-description

The current SPDXmerger accepts any content for this field as input and replicates it in the output document. The program should rather check for syntax compliance before merging the document, refuse to merge if not compliant, with an error message that allows to fix the issue and understand the expectation.