Anisotropic gradient

• Simulation: gradient, jvec, and jtvec new support triaxial
  anisotropy (also through the CLI). As a consequence, gradient and
  jtvec return an ndarray of shape (nx, ny, nz) (isotropic) or
  ({2;3}, nx, ny, nz) (VTI/HTI; triaxial), and jvec expects an ndarray
  of shape (nx, ny, nz) (isotropic) or ({1;2;3}, nx, ny, nz)
  (isotropic; VTI/HTI; triaxial).