Write Python version of filter.R to parse Torperf's new .mergedata format

Once we have completed the improved consolidate_stats.py script in #2672 (closed), we should update our filter.R script to process the new .mergedata format instead of .data files. We could start with parsing the 20 fields in .mergedata coming from .data files. Once we have that under control, we might extend filter.R to parse new fields coming from .extradata files. See my comment to #2618 for an implementation idea.