Update CKAN Schema.org output to follow ESIP Science-on-Schema.org recommendations

[mwengren]

ESIP recently released version 1.2.0 of their Science-on-Schema.org guidelines. SoSO guidelines for Datasets are available here:

https://github.com/ESIPFed/science-on-schema.org/blob/master/guides/Dataset.md

Many of their recommendations we won't be able to comply with easily in the IOOS Catalog because we don't have that level of detail in our source metadata, but there's at least one or two easy fixes we can make that will also help our results in Google's Rich Results Test tool - example IOOS dataset results for this NANOOS dataset.

1. Add contentURL to all distribution elements: CKAN by default attaches a url attribute here, but both Google and SoSO recommend contentURL. See: https://github.com/ESIPFed/science-on-schema.org/blob/master/guides/Dataset.md#distributions

2. Add isAcessibleForFree: true to all datasets. All data in IOOS Catalog should be freely accessible. This is a Google recommendation but also referenced by SoSO.

3. SoSO and the Google Rich Results Test both recommend including creator field in a Dataset. SoSO guidelines for that are here: https://github.com/ESIPFed/science-on-schema.org/blob/master/guides/Dataset.md#roles-of-people. Because creator appears to be a person, and not an institution, we would have to source this from the CI_ResponsibleParty elements in the ISO XML. For example, the NANOOS ROMS Oregon Coast dataset includes a few with role=Originator. According to this ESIP CI_RoleCode page, any of the following can map to the DataCite 'creator' concept: author, originator, principalInvestigator, resourceProvider. Assuming DataCite and SoSO have the same concept of 'creator', can we map any ISO XML CI_ResponsibleParty elements with CI_RoleCode matching one of the above to a Schema.org creator element following the SoSO guidelines? It appears multiple creators are possible.