Spm exporter

[hermann74]

• Fix bugs in SPM exporter to follow BEP028_BIDSprov specifications
• add unitary test and CI in github actions ( compare jsonld result as graph)
• refactor codes of spm_exporter
• document code of SPM exporter
• change pointer of context url to master branch

[cmaumet]

Thanks @hermann74 and @cyril-data ! Can you look into why the checks failed and propose a patch?

[cmaumet]

Thank you @hermann74 @cyril-data