Add TTM method

[luowh35]

Summary
This PR resolves issue #1427 by adding two-temperature-model (TTM) support to GPUMD for thermal-transport simulations involving metallic systems. It introduces both a pure ensemble ttm mode and a ensemble heat_ttm mode that combines source/sink Langevin regions with the electron temperature field. The implementation also includes electron temperature output and optional per-cell electron properties.

Modification

• Add a new Ensemble_TTM integrator in:
  • src/integrate/ensemble_ttm.cuh
  • src/integrate/ensemble_ttm.cu
• Add support for two new ensemble commands:
  • ensemble ttm
  • ensemble heat_ttm
• Extend Integrate parsing and initialization in:
  • src/integrate/integrate.cu
  • src/integrate/integrate.cuh

Others

[luowh35]

Detailed documentation is required for use, coming soon.