Utilize Java 8 in RFile

There are a few places in o.a.a.core.file.rfile.RFile where Java 8 functional programing could improve readability. There is some potential for performance improvement as well.

One example of an improvement would be to change the 3 LocalityGroupReader arrays (currentReaders, readers and sampleReaders) in the RFile.Reader class to ArrayLists. This would allow use of the forEach in stream package.