Investigate alternative implementations for SUBSET_READS_PAIRED_TARGET

[willbradshaw]

SUBSET_READS_PAIRED_TARGET is currently a bit of a performance bottleneck. It's currently implemented with seqtk; since it's running on the entire dataset it's possible this is just irreducibly slow, but it's worth investigating whether alternative tools could do the same thing while lopping off some runtime.

However, this is lower priority than other performance enhancements included as issues here, e.g. #127, #128, #122, #74.