Fixed crash in PTflash_twophase::build_arrays

[tobias-loew]

Fixed crash in SaturationSolvers::PTflash_twophase::build_arrays for mixtures with 3 or more components.

Requirements

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• All new code requires tests to ensure against regressions.

Description of the Change

changed used index

Benefits

code crashed hard because of wrong index

Possible Drawbacks

Verification Process

Please check, if numerical properties of algorithm are now correct

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• How did you verify that the change has not introduced any regressions?

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Applicable Issues

[tobias-loew]

There is still the problem in src/Backends/Helmholtz/FlashRoutines.cpp that with phase-envelope built ONLY single phase calculation are supported. This seems weird because WITHOUT phase-envelope two-phase calculation are at least tried!