Linked data APIs

Fixes Fixes for #126 (closed)

Description Set up APIs for linked data.

Type of PR This PR is a feature.

Technicalities

• LinkedDataComponent handles content negotiation
• Entities have a getCidocCrm() function which returns linked data (mainly using the CIDOC-CRM ontology) in parsed JSON-LD format
• A number of tables have additional associations defined: composite ←→ witnesses, seal ←→ impressions, artifact → place of origin (written_in), artifact → fragment (artifact can consist of multiple materials)
• Entity/LinkedDataTrait houses helper functions for creating JSON-LD
• Views for JSON-LD, RDF/JSON, RDF/XML, N-Triples and Turtle are added
• View/MetadataExportTrait houses helper functions for converting linked data formats (using EasyRDF for formatting)
• Documentation pages are added for the various APIs (the linked data format still needs documentation)

Tests

• Request http://127.0.0.1:2354/proveniences/2 with HTTP header Accept: text/turtle
• Request http://127.0.0.1:2354/materials with HTTP header Accept: application/n-triples
• Request http://127.0.0.1:2354/P000001 with HTTP header Accept: application/ld+json
• Documentation at http://127.0.0.1:2354/docs/api
• Meta-ontology at http://127.0.0.1:2354/docs/vocab/1.0

Checklist:

• My pull request has a descriptive title (not a vague title like "Update index.md").
• My pull request targets the phoenix/develop branch of the repository.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.