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) + Math.pow(a[6], 2) + Math.pow(a[7], 2) + Math.pow(a[8], 2));
}
Returns Frobenius norm of a mat3

@param {mat3} a the matrix to calculate Frobenius norm of
@returns {Number} Frobenius norm