[tdwg-humboldt] Use case representing absences and the Humboldt Extension

Hi all (absence use case working group + Humboldt working group), John has developed the Broke west trawl dataset mini use case into a full use case representing absences and the Humboldt Extension and can be found ready for review at https://docs.google.com/document/d/1DgYrSlCCcvs4x59J8Koty6TTovRKtQhAYe8ItE0g... Thank you so much John for working on this! YAYYYY!! Please feel free to review this! I will make time after tomorrow to have a closer look at this as well! Thank you all! Cheers Ming