function frob(a) {
  return Math.sqrt(Math.pow(a[0], 2) + Math.pow(a[1], 2) + Math.pow(a[2], 2) + Math.pow(a[3], 2) + Math.pow(a[4], 2) + Math.pow(a[5], 2) + 1);
}
Returns Frobenius norm of a mat2d

@param {mat2d} a the matrix to calculate Frobenius norm of
@returns {Number} Frobenius norm