Add support for ODIS fields to json-ld output

[fostermh]

ODIS Instructions: https://book.oceaninfohub.org/thematics/dataset/index.html

From my initial investigation:

• [x] 1. Licence: CIOOS uses a URL for this value, ODIS is a text string. Is a URL acceptable?
  • we will use url
• [x] 2. version: in CIOOS this is a number, should it be a date?
  • we will use version field if available and fall back to most recent metadata-reference-date if not. finily use 1 if nothing elsel available.
• [x] 3. citation: does ODIS support other formats than the APA string? It would be very convenient for us if we could send json, for example.
  • leaving blank as this field is to record publications / datasets that use the dataset. might populate in future using pid search?
• [x] 4. ID: CIOOS uses the url to the dataset keyed by DB pk. ODIS suggests using json_ld document as target, is ours ok?
• [x] 5. SubjectOf: how do we fill it out?
  • should be 'about' leave blank for now. we do not have info to populate this field

Things we currently do not capture within CKAN and possible CIOOS

1. variableMeasured
2. measurementTechnique
3. subjectOf

[fostermh]

based on this supporting document it appears a URL in the license field is acceptable. https://book.oceaninfohub.org/thematics/license/README.html

[fostermh]

see https://github.com/iodepo/odis-arch/pull/127/files#diff-d53c0e2247fda6217510486f4b522eaafc49ef325d76bf46a8baa6e9b10bd7b2 for example of ways to handle variables measured. ideal if we can have urls for the veriabls, CF names would work. eov's can fit under this as well. see https://book.oceaninfohub.org/thematics/variables/index.html

[fostermh]

SubjectOf in dataset example in ODIS book is wrong, should be 'About'. We likely do not have the info to populate this anyway.

@ID should be json_ld document. how to change in DCAT extension? I think it's baked into the core code.

Citation field is for citing things that use this dataset. in our case we will leave this blank. might be able to populate using PID search in the future.

Use metadata published / revised date for version.

url in license is ok if that is what we already have, better to use string as URL is not guaranteed to persist.

[fostermh]

done and merged into cioos_dev. will be in next release