core: use physics path in etcs braking simulator

[Erashin]

PhysicsPath gives access to gradient AND can be accessed in this part of the code.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 38.19%. Comparing base (02ccfc8) to head (3d0485e).
Report is 1 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #9937      +/-   ##
==========================================
- Coverage   38.21%   38.19%   -0.02%     
==========================================
  Files         992      992              
  Lines       92158    92158              
  Branches     1186     1186              
==========================================
- Hits        35221    35203      -18     
- Misses      56486    56504      +18     
  Partials      451      451              

Flag	Coverage Δ
editoast	73.33% <ø> (-0.06%)	⬇️
front	20.18% <ø> (ø)
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	87.00% <ø> (ø)

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