Introduced bfmerge script

The new bfmerge script inputs a set of Byfl output files produced from
multiple runs of the same program or from a single, parallel run using
BF_PREFIX.  bfmerge then re-outputs each line of input, replacing each
number with the mean value across all files and likewise for other
statistics (minimum, maximum, median, sum, standard deviation, and
median absolute deviation).  This process enables investigations of
variability across runs or across processes within a run.