Fix issues with 1D flame solver

[speth]

Changes proposed in this pull request

This PR fixes a number of issues with the 1D flame solver.

• Handle empty string as transport model more consistently. This now always means the none transport model. The string default needs to be specified to activate the YAML-specified transport model. Resolves #1680
• Detect invalid boundary conditions for flame configurations. Fixes #1694
• Eliminate redundant calculation of stoichiometric mixture fraction in diffusion flame initial guess
• Disallow redundant flame grid specification. Fixes #1693
• Fix setting diffusion flame initial guess from known data. Fixes #1674
• Handle potential exceptions propagating from callbacks when switching thermo/kinetics/transport objects attached to a Solution
• Automatically adjust temperature bounds for low-temperature flames. Fixes #1684
• Tweak flame tests to run faster. Previously, the single test TestCounterflowPremixedFlameNonIdeal::test_solve_case1 was previously taking upwards of 30 seconds to run on GitHub Actions.

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.43590% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.72%. Comparing base (026c5c7) to head (790dd3a).

Files	Patch %	Lines
interfaces/cython/cantera/onedim.py	95.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1705 +/- ## ========================================== + Coverage 75.69% 75.72% +0.02% ========================================== Files 443 443 Lines 60971 60981 +10 Branches 9552 9557 +5 ========================================== + Hits 46154 46178 +24 + Misses 11786 11777 -9 + Partials 3031 3026 -5 ```

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[speth]

I'm going to leave the Windows .NET CI failure alone for the sake of this PR. I tried testing locally and can't replicate it.