Add resources (classes) to the JSON-LD context.

[jesper-friis]

Description

From the data documentation in COLD we learned that it is useful to also include the classes in the JSON-LD context.

This makes it e.g. possible to short names when referring to classes in JSON-LD files. Ex: {"@type": "Resource", ...}

This PR is also used in PR #360 to replace keywords with JSON-LD context in many parts of the dataset API. The benefit with this is that this allows to use existing externally defined JSON-LD contexts.

Also added a __dir__() method to Keywords that include the documented attributes. It simplifies interactive exploration of Keyword instances using ipython (used while implementing this PR).

Type of change

• Bug fix and code cleanup
• New feature
• Documentation update
• Testing

Checklist for the reviewer

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?
• Is the code properly tested?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.46%. Comparing base (aabd5ec) to head (681b326).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
+ Coverage   80.43%   80.46%   +0.02%     
==========================================
  Files          25       25              
  Lines        2970     2974       +4     
==========================================
+ Hits         2389     2393       +4     
  Misses        581      581              

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