Bugs of `dfSteadyFoam` using `flareFGM` combustion model

[TimoLin]

Some bugs related to the flareFGM model in steady state solver.

Test case: twoD_SandiaD_flareFGM from examples, the settings has been adapted according to the steady solver dfSteadyFoam.

1. ddt term

ddt term of each flamelet variable's equation should be removed foir the steady solver https://github.com/deepmodeling/deepflame-dev/blob/1be82b637cd03ddcdbff61a140385c8a824a5ee2/src/dfCombustionModels/FGM/baseFGM/baseFGM.C#L778-L789

2. Treatment of chi_zc for 4D table

If the table only has 4 dimensions (Z, Zvar, C, Cvar), then the contribution of Chi_ZC shouldn't be added to the source term of C, either set Chi_ZC value to 0 or don't add it to the omega_c term. https://github.com/deepmodeling/deepflame-dev/blob/1be82b637cd03ddcdbff61a140385c8a824a5ee2/src/dfCombustionModels/FGM/flareFGM/flareFGM.C#L296-L305

3. relaxation of flamelet variable equations

The relaxation_ switch seems to be duplicated and may cause misunderstanding. https://github.com/deepmodeling/deepflame-dev/blob/1be82b637cd03ddcdbff61a140385c8a824a5ee2/src/dfCombustionModels/FGM/baseFGM/baseFGM.C#L518-L536 If user want's to apply relaxation to Z, the following settings should be both set in constant/combustionProperties and system/fvSolution:

// combustionProperties
flareFGMCoeffs
{  
...
  relaxation       true;
...
}

// fvSolution
relaxationFactors
{
...
    equations
    {
...
        "(Z|Zvar|c|cvar|Zcvar|Ha).*"                       0.5;
...
    }
}