add TGSimple model

[rodrigomha]

Add basic single state turbine governor struct

τ = P_ref + d_t * (ω_ref - ω)
d τm / dt = (τ - τm) / Tm

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.62%. Comparing base (7fc254a) to head (509f07a).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/models/generated/TGSimple.jl	83.33%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1195      +/-   ##
==========================================
- Coverage   84.63%   84.62%   -0.01%     
==========================================
  Files         180      181       +1     
  Lines        8264     8282      +18     
==========================================
+ Hits         6994     7009      +15     
- Misses       1270     1273       +3     

Flag	Coverage Δ
unittests	84.62% <83.33%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
src/PowerSystems.jl	100.00% <ø> (ø)
src/models/generated/TGSimple.jl	83.33% <83.33%> (ø)