Counterflow flame initial condition

[wandadars]

When running a counterflow diffusion flame case with two inert boundaries, Cantera fails to run because it attempts to use a stoichiometric initial condition that depends on elements that may not be present in the flow. This adds a mode= to the set_initial_guess() method to allow someone to simply set a linear interpolation between BCs as the initial solution guess. A unit test for the new mode= option was also added to test the double inert BC solve.

AI Statement (required)

• Extensive use of generative AI.
  Significant portions of code or documentation were generated with AI, including
  logic and implementation decisions. All generated code and documentation were
  reviewed and understood by the contributor. Examples: Output from agentic coding
  tools and/or substantial refactoring by LLMs (web-based or local).

For additional information on Cantera's AI policy, see
https://github.com/Cantera/cantera/blob/main/CONTRIBUTING.md#restrictions-on-generative-ai-usage -->

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.35%. Comparing base (f49df44) to head (082fcde).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2083      +/-   ##
==========================================
+ Coverage   77.34%   77.35%   +0.01%     
==========================================
  Files         451      451              
  Lines       52494    52514      +20     
  Branches     8847     8851       +4     
==========================================
+ Hits        40602    40624      +22     
+ Misses       8917     8916       -1     
+ Partials     2975     2974       -1     

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