Fix deprecation warnings

[jesper-friis]

Description

Fixed some deprecation warnings.

• Replace calls to deprecated functions: get_jsonld_context(), get_prefixes() and get_shortnames().
• Updated accepted arguments of get_context() (and underlying methods in Context) to enable the replacement of get_jsonld_context().
• We keep for now the tests of the deprecated functions get_jsonld_context(), get_prefixes() and get_shortnames(), but ignore deprecation warnings within them.
• Pint UnitRegistry has replaced index access to function access, i.e. ureg[] -> ureg().

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 84.05%. Comparing base (190475e) to head (5e9d232).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
+ Coverage   83.98%   84.05%   +0.06%     
==========================================
  Files          30       30              
  Lines        4140     4151      +11     
==========================================
+ Hits         3477     3489      +12     
+ Misses        663      662       -1     

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