Proposal: Change positions code

[ayacoo]

getProductStartLine() and getProductEndLine() are private functions and used in getPositions().

The parser exception is already thrown in the getPositions() function. So I would remove this in the private functions and return a 0 in the worst case. getPositions() "loops" from 0 to 0 and throws the ReceiptParseException. So we can test this case with different number of positions.