Re-wrote MeterGroup.select_top_k().  Closes #142

@nipunreddevil: I think your function was written before we wrote a
bunch of useful helper functions.  So now we can implement
select_top_k in only a few lines.  Please do let me know if I have
missed something!  Also, I agree we should ultimately make
select_top_k more general, but, for now, I think this is sufficient???