[1D] Bugfix : Segfault if no transport specified in the YAML file

[joebarteam11]

call "registerChangedCallback" before transport setting if transportModel() is "none"

Changes proposed in this pull request

• Segfault encountered if no "transport:" field specified in the YAML file
• Calling registerChangedCallback before defining "mixture-averaged" as default transport model in StFlow constructor solves the problem

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Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review

[ischoegl]

As an aside: the patch was for 3.0, rather than main. The offending code is no longer present in main; at this point, the fix will apply for those who compile 3.0 from source.