Handle multiple type codes in 03 (Account Identifier) records

[barszcz]

Bai2 Version: 0.1.0

What were you trying to do?

What did you expect to see?

What did you see?

How can we reproduce the problem?

03 (Account Identifier) records in BAI2 may contain sets of (type code, amount, item count, funds type) records multiple times in sequence for an arbitrary number of type codes. Currently, the library treats this sequence as though it can only occur once per record. (Frustratingly, this very important detail about the format is buried in a confusingly worded footnote on page 16 of the spec and is otherwise left unclear.) Once we're returning the "fully parsed" file model from #52 we can enhance the account object to contain a slice of these.