Add GERG-2008-H2 Model with Improved Hydrogen Parameters

Summary

Implements the GERG-2008-H2 equation of state extension with improved hydrogen binary interaction parameters based on Beckmüller et al. (2022).

Changes

• New GERG2008H2 class - Extends GERG2008 with updated hydrogen parameters for CH₄-H₂, N₂-H₂, CO₂-H₂, and other binaries
• New GERG2008Type enum - STANDARD and HYDROGEN_ENHANCED model selection
• Updated SystemGERG2008Eos - Added useHydrogenEnhancedModel() and setGergModelType() methods
• Updated NeqSimGERG2008 - Constructor accepts model type parameter
• Documentation - Added GERG-2008-H2 section to docs/thermo/gerg2008_eoscg.md

Usage

SystemGERG2008Eos fluid = new SystemGERG2008Eos(300.0, 50.0);
fluid.addComponent("methane", 0.7);
fluid.addComponent("hydrogen", 0.3);
fluid.useHydrogenEnhancedModel();  // Enable GERG-2008-H2

Key Differences from GERG-2008

Reference

Beckmüller, R. et al. (2022). "Extension of the equation of state for natural gases GERG-2008 with improved hydrogen parameters". Fluid Phase Equilibria, 557, 113411.