Enhanced the use of namespaces

[jesper-friis]

Description

Simplified and improved the usability of namespaces

• Added persistent cache to Namespace. Loading a large namespace like EMMO from cache is now almost instant.
• Added extend_namespace() utility function.
• Simplified the implementation of Namespace, including:
  • replaced cachemode argument with the simple boolean argument reload
  • removed triplestore_url argument
  • added format argument for supporting loading ontologies in formats other than turtle
• Updated the Triplestore.bind() method to make it simpler to use.
• Updated the tutorial and improved the description of how namespaces can be used.
• Added and improved tests. Added example for simplified SPARQL queries using tripper Namespace.

Closes #194

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 87.85047% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 78.43%. Comparing base (6500dca) to head (d3bc81a).

Files	Patch %	Lines
tripper/namespace.py	83.54%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
+ Coverage   78.42%   78.43%   +0.01%     
==========================================
  Files          18       18              
  Lines        1599     1651      +52     
==========================================
+ Hits         1254     1295      +41     
- Misses        345      356      +11     

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[francescalb]

Nice, I especially like to better introduction. However, there are quite a few lines not tested. I think these should be added.

[jesper-friis]

Nice, I especially like to better introduction. However, there are quite a few lines not tested. I think these should be added.

Added some more tests, but:

• No tests are added for the deprecated arguments in Namespace.init() (they would have to be removed soon again).
• Windows- and Mac-specific stuff in get_cachedir() are not testet either.

[francescalb]

Why remove this warning?

[francescalb]

Nice, but please explain in the Docstring why base_iri is a valid Argument