Enable ISO9660 to be subclassed with an overridden get_sector method

I need to subclass ISO9660 to support retrieving the sectors from the file using another method.  This pull request includes two small changes to make this possible/easier:

1. 74c7538 provides a constant for the sector size so that the subclass can reference this instead of having to use the magic number "2048".

2. 74c7538 allows the _get_sector member to be overridden by subclasses.

Thanks for providing this code!