frobeniusPullback -- compute Frobenius pullbacks

• Usage:

  frobeniusPullback(e, M)

• Inputs:
  • e, an integer,
  • M, a ring, an ideal, a module, a matrix, a coherent sheaf, or a morphism of sheaves,
• Outputs:
  • a ring,
  • a module,
  • a matrix,
  • a coherent sheaf,
  • a morphism of sheaves,