core: stdcm: fallback to linear envelope

[eckter]

Fix #5196

[codecov]

Codecov Report

Merging #5330 (50c3ee3) into dev (087973a) will decrease coverage by 0.01%.
Report is 2 commits behind head on dev.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##                dev    #5330      +/-   ##
============================================
- Coverage     18.73%   18.73%   -0.01%     
  Complexity     2308     2308              
============================================
  Files           849      849              
  Lines        103050   103055       +5     
  Branches       2390     2391       +1     
============================================
- Hits          19307    19306       -1     
- Misses        82411    82416       +5     
- Partials       1332     1333       +1     

Flag	Coverage Δ
core	78.69% <33.33%> (-0.05%)	⬇️

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Files	Coverage Δ
...a/fr/sncf/osrd/reporting/exceptions/ErrorType.java	100.00% <ø> (ø)
.../sncf/osrd/stdcm/graph/STDCMStandardAllowance.java	71.42% <33.33%> (-8.58%)	⬇️