This is in response to a request from Mauricio to make it easier
to use the downsamplers with GATKSAMRecords (as opposed to SAMRecords)
without having to do any cumbersome typecasting. Sadly, Java
language limitations make this sort of solution the best choice.
Thanks to Khalid for his feedback on this issue.
Also:
-added a unit test to verify GATKSAMRecord support with no typecasting required
-added some unit tests for the FractionalDownsampler that Mauricio will/might be using
-moved classes from private to public to better sync up with my local development
branch for engine integration
David Roazen