Make it possible to expose an existing rdflib graph via tripper

[jesper-friis]

Description:

Added graph option to the rdflib backend that allows to expose an existing rdflib graph via tripper.

Use as follows:

graph = Graph()
graph.parse(...)
ts = Triplestore(backend="rdflib", graph=graph)  # all triples in the original graph are now exposed via tripper

Type of change:

• Bug fix.
• 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?

[codecov-commenter]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (dba3971) 75.67% compared to head (ddd3bec) 75.17%.
Report is 49 commits behind head on master.

Files	Patch %	Lines
tripper/mappings/mappings.py	40.00%	9 Missing ⚠️
tripper/backends/rdflib.py	80.00%	1 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
- Coverage   75.67%   75.17%   -0.51%     
==========================================
  Files          17       17              
  Lines        1402     1410       +8     
==========================================
- Hits         1061     1060       -1     
- Misses        341      350       +9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[francescalb]

Approved with fixing of a typo