Use Math::BigFloat

Modified bfmerge to use Perl's Math::BigFloat library to avoid
numerical overflows when merging lots of Byfl output files, each
containing very large numbers.
Added a --parse option

bfmerge can now selectively process input lines based on a regular
expression provided via the --parse option.  For example,
--parse=BYFL_SUMMARY tells bfmerge to examine only input lines
containing the string "BYFL_SUMMARY".
Reduced memory consumption

Localized bfmerge's use of Math::BigFloat so statistics are computed
on (space-hungry) bignums, and then those bignums can be safely
garbage-collected.