Add initialization option for incompressible solver

[Cristopher-Morales]

Proposed Changes

This pull request aims to implement initialization option for the incompressible solver similar to the one for the compressible solver.
For some applications using fluid mixture and fluid flamelet model, using operating pressure is preferred. Other applications, the operating pressure is computed using the INC_DENSITY_INIT and INC_TEMPERATURE_INIT options. So, this implementation aims to have a better distinction between both ways of initialization.

Related Work

This was an old pull request #2137

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