Add sampling methods to allow:
-preserving mate/frag ratio
-only mated reads
-breaking mates (only way to get a mate pair is by picking both reads independently)
What is currently implemented is a small set of special cases. Is it possible to make this both simple and general?
-samplefrags -- pick reads independently
-samplemates -- pick mates independently
-includeunmated -- for -samplemates, also allow unmated reads