Update SAREF alignment

[dr-shorthair]

1. sosa:observedProperty is no longer in SSN/SOSA
2. are there any other additions required as a consequence of the Asset class and associated properties? https://w3c.github.io/sdw-sosa-ssn/ssn/#Systems-and-their-Deployment-overview

[dr-shorthair]

@maximelefrancois86 Can you confirm that the SAREF alignment survived the more recent changes, esp. around Asset ?
https://w3c.github.io/sdw-sosa-ssn/ssn/#SAREF-alignment