function mod (x1, x2) {
  return NdArray.new(x1).mod(x2);
}
Return element-wise remainder of division.
Computes the remainder complementary to the `floor` function. It is equivalent to the Javascript modulus operator``x1 % x2`` and has the same sign as the divisor x2.

@param {(NdArray|Array|number)} x1
@param {(NdArray|Array|number)} x2
@returns {NdArray}