Change base iri for sphn
This PR changes the base IRI for SPHN from https://biomedit.ch/rdf/sphn-ontology/sphn#
to https://biomedit.ch/rdf/sphn-schema/sphn#
The changes have been made in a backwards compatible manner such that existing Dataset excel that is still on old IRIs would still parse without any errors.
Dataset2rdf now solely relies on the config.yaml (and the 'Metadata' sheet in the Dataset excel) for the base (canonical) IRI.