code: def from_index_amount(cls, matrixpos, amt):
          
          f = np.identity(3)
          f[matrixpos] += amt
          return cls(f)
docs: Factory method for constructing a Deformation object
          from a matrix position and amount
  
          Args:
              matrixpos (tuple): tuple corresponding the matrix position to
                  have a perturbation added
              amt (float): amount to add to the identity matrix at position
                  matrixpos