Improved the handling of different return types from the query() method

[jesper-friis]

Description

Improved and documented the handling of different return types from the query() method.

Note that not all lines that codecov complains about are possible to test simultaneously, because the SPARQL interface in tripper.backends.rdflib supports multiple versions of rdflib.

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 64.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 78.34%. Comparing base (cbcccab) to head (c6811b3).

Files	Patch %	Lines
tripper/backends/rdflib.py	62.50%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
- Coverage   78.46%   78.34%   -0.13%     
==========================================
  Files          18       18              
  Lines        1672     1690      +18     
==========================================
+ Hits         1312     1324      +12     
- Misses        360      366       +6     

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[francescalb]

Looks fine, but should we maybe have tests for the different types of queries?
SELECT, ASK, CONSTRUCT and DESCRIBE queries.