function AuxPow(header, data) {
  if (!(this instanceof AuxPow)) {
    return new AuxPow(header, data);
  }

  $.checkArgument(header && header.version, 'version is missing from header')
  if (!(header.version & (1 << 8))) {
    return;
  }

  const info = this._from(data);
  this.coinbaseTxn      = info.coinbaseTxn;
  this.coinbaseBranch   = info.coinbaseBranch;
  this.blockchainBranch = info.blockchainBranch;
  // Note: the blockhash is only used when re-serializing. Otherwise, use the parent block hash
  this._blockHash       = info.blockHashBuf;
  this._parentBlock     = info.parentBlockBuf;

  return this;
}
Parse the Aux Proof-of-Work block in the block header
Ref: https://en.bitcoin.it/wiki/Merged_mining_specification#Aux_proof-of-work_block
@param {BlockHeader} header - BlockHeader this is attached to
@param {BufferReader} br - BufferReader containing the header