v6.0.0

• Inelastic materials are written on a contract (breaking): a
  Models.InElastic._Behavior declares its Internals and writes Update(eps, z, dt)
  and Stress(eps, z) at one 3D point in jax.numpy; EasyFEA lifts them to every Gauss
  point, derives the tangent by jax.jacfwd, handles plane strain and plane stress, and
  commits the internal state at every converged Solve (Save_Iter only records it, so
  it can be skipped on some steps). Newton, Trace, Deviator, Von_Mises_stress
  help write one. A behavior takes only its elastic model, _Behavior(elastic): dim,
  planeStress and thickness are the model's, C its 3D stiffness even in 2D, and
  modifying the model rebuilds the kernel. External variables, such as a temperature,
  are declared as Externals and set with simu.Set_external(T=...).
• Shipped behaviors: Plasticity(elastic, surface, hardening), Norton, Chaboche
  with N back-stresses, and Maxwell with N branches. Surfaces (Yield.VonMises,
  Hill, DruckerPrager) and hardening (IsotropicHardening.Perfect, Linear,
  Voce, Swift) are plain callables.
• The numpy Behavior(dim, elastic, yieldSurface, hardening, kinematic, rate, branches)
  is removed, with KinematicHardening, ViscoPlastic (Perzyna is Norton with
  A = 1/eta) and ViscoElastic (breaking). Chaboche's back-stresses are one
  alpha of shape (N, 6), not alpha0, alpha1, ...; Simulations.InElastic no
  longer reports a free energy.
• Inelastic behaviors need jax: pip install EasyFEA[jax]. The README lists the
  optional dependencies by extra (viz, io, jax).
• Boundary surfaces of 3D meshes built with the OCC factory are oriented outward, so
  their normals may flip sign.
• Fixed the weights of the 6-point triangle quadrature rule, truncated to about 14
  digits.

Full Changelog: v5.0.0...v6.0.0