Minor cleanup of operator stuff in TransitModel

[habrahamsson-skanetrafiken]

Summary

• Add specific method for adding operators.
• Make operator list unmodifiable.
• Remove TransitService.getOperators() since we already have TransitService.getAllOperators().

Issue

#6048

Unit tests

No tests for this.

Bumping the serialization version id

I'm changing the signature of TransitModel.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.83%. Comparing base (9e58db4) to head (014824b).
Report is 127 commits behind head on dev-2.x.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6143      +/-   ##
=============================================
- Coverage      69.85%   69.83%   -0.02%     
- Complexity     17491    17665     +174     
=============================================
  Files           1976     1995      +19     
  Lines          74712    75269     +557     
  Branches        7645     7700      +55     
=============================================
+ Hits           52192    52567     +375     
- Misses         19868    20028     +160     
- Partials        2652     2674      +22     

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