Enable alternative material models for triangle/tetrahedra mesh

[ShaoxiongYao]

Hi,

Thank you for providing this library!

I want to ask if there is a plan to enable alternative material models in integrator_euler.py. The current tetrahedra mesh assumes the Neo-Hookean model and alternative models have been implemented but commented out. For example, in eval_triangles there is alternative St. Venant-Kirchoff model implemented. I wonder if there will be flags to enable different material models in later iterations.