Added documentation for datasets

[jesper-friis]

Description

Added documentation for datasets. Other changes:

• added utils.openfile() and replaced open() with utils.openfile() in TableDoc.parse_csv()
• import backends in tripper.__init__.py to avoid pytest failing with RuntimeError: dictionary changed size during iteration when collecting tripper.__init__.py.
• several other minor changes added while debugging the above issue

The documentation can be checked locally with:

pip install -U -e .[docs]
mkdocs serve
# Open http://127.0.0.1:8000/tripper/ in your browser

Builds on top of PR #279

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

Attention: Patch coverage is 97.77778% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.18%. Comparing base (6101c87) to head (e054557).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
tripper/utils.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
+ Coverage   78.69%   79.18%   +0.49%     
==========================================
  Files          21       21              
  Lines        2229     2263      +34     
==========================================
+ Hits         1754     1792      +38     
+ Misses        475      471       -4     

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[torhaugl]

Mostly improved documentation. I think a link to JSON-LD playground would be nice in the customization part. I'll let you decide if you want to commit these changes before merging.

I did not read all the descriptions for the pre-defined keywords in detail, but a quick glance and they look reasonable. Includes a new function openfile (to open over http) and corresponding test.