isCoinbase method
Implementation
bool isCoinbase() {
//if we have a Transaction with one input, and a prevTransactionId of zeroooos, it's a coinbase.
return (_txnInputs.length == 1 &&
(_txnInputs[0].prevTxnId == null ||
_txnInputs[0].prevTxnId.replaceAll('0', '').trim() == ''));
}