Don't expose mutable tripOnServiceDates in TransitModel

[habrahamsson-skanetrafiken]

Summary

Just a minor cleanup to not expose a mutable map and instead provide TransitModel.addTripOnServiceDate()

Encapsulation, loose coupling etc.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 67.91%. Comparing base (5b3be61) to head (07aab67).
Report is 27 commits behind head on dev-2.x.

Files	Patch %	Lines
...in/java/org/opentripplanner/netex/NetexModule.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #5842      +/-   ##
=============================================
+ Coverage      67.90%   67.91%   +0.01%     
+ Complexity     16567    16561       -6     
=============================================
  Files           1910     1910              
  Lines          72418    72439      +21     
  Branches        7444     7444              
=============================================
+ Hits           49173    49199      +26     
+ Misses         20724    20720       -4     
+ Partials        2521     2520       -1     

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[t2gran]

This looks good. I checked, dated trips are only added during NeTEx graph build. It is potensialy dangerous to use during routing.