Support __getitem__ for Groups

[PythonFZ]

Fixes #773 by adding support for

assert list(group_1) == [node_1, node_2]
group_1["ParamsToOuts"] == node_1
group_1["Group1_ParamsToOuts"] == node_1
"Group1_ParamsToOuts" in group_1
"ParamsToOuts" in group_1

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ce2a572) 86.60% compared to head (cec1cbe) 86.69%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #774      +/-   ##
==========================================
+ Coverage   86.60%   86.69%   +0.09%     
==========================================
  Files          25       25              
  Lines        2015     2030      +15     
  Branches      856      867      +11     
==========================================
+ Hits         1745     1760      +15     
  Misses        270      270              

Flag	Coverage Δ
fulltest	86.69% <100.00%> (+0.09%)	⬆️

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