Document the Hertz-Mindlin contact model and its time step

Jordan Musserrequested to merge
jmusser/exa-docs:feature/dem-hertz into main
  • DEM model: the dem.hertz.* inputs (Young's modulus and Poisson ratio per type and for the walls, tangential damping factors), with an example.
  • Time stepping: what tcoll and tcoll_ratio mean under each model (collision time for lsd, Rayleigh time for hertz), and the tcoll_ratio default per model: 10 with hertz, 50 otherwise. The new label InputsDEMTimeStep points at that section.
  • Adds the Tsuji et al. (1992) reference.

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