FIX: issue 1939 fix grpc Terminal not set as reference causing random failure on SIwave

[svandenb-dev]

This PR is addressing issue #1939 found by a user where some methods have hard coded is_ref=False. These terminals are functional but according to the users, this is causing random failure when importing EDB in Siwave. Some voltage source or Current source might be ignored during import. Making sure that negative terminal is always set as reference is resolving this failure.

closes #1939

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.94%. Comparing base (5cbae21) to head (ac8d5a5).
⚠️ Report is 43 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1944      +/-   ##
==========================================
+ Coverage   70.20%   70.94%   +0.73%     
==========================================
  Files         261      262       +1     
  Lines       42416    42796     +380     
==========================================
+ Hits        29779    30362     +583     
+ Misses      12637    12434     -203     

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