function _matrixNormFrobenius (x) {
      // norm(x) = sqrt(sum(diag(x'x)))
      let fro = 0
      x.forEach(function (value, index) {
        fro = add(fro, multiply(value, conj(value)))
      })
      return abs(sqrt(fro))
    }
Calculate the Frobenius norm for a matrix
@param {Matrix} x
@returns {number} Returns the norm
@private