[DRAFT] [PSY5] Add 3W Transformer Struct

[rodrigomha]

Initial idea for 3W transformer based on PSSE.

We are not giving support to any tap voltage control (i.e. COD1 = COD2 = COD3 = 0), and we are assuming that all the data must come in system per unit. These requires the flags CW = CZ = CM = 1.

[codecov]

Codecov Report

Attention: Patch coverage is 97.18310% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.41%. Comparing base (effc91c) to head (4521353).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/models/generated/Transformer3W.jl	97.18%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1247      +/-   ##
==========================================
+ Coverage   84.30%   84.41%   +0.10%     
==========================================
  Files         181      182       +1     
  Lines        8409     8480      +71     
==========================================
+ Hits         7089     7158      +69     
- Misses       1320     1322       +2     

Flag	Coverage Δ
unittests	84.41% <97.18%> (+0.10%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/PowerSystems.jl	0.00% <ø> (ø)
src/models/generated/Transformer3W.jl	97.18% <97.18%> (ø)