Changed the way blast2graph.py handles zero E-values

Instead of just setting zero E-values to 1e-181 (an order of magnitude less
than the minimum score output by BLAST) and then scaling those along with the
rest of the E-values, blast2graph.py now uses the proxy values to calculate
inter- and intra- organism averages, scales the proxy values along with the
rest, and then shifts them down so that the largest proxy value is still an
order of magnitude lower than the smallest scaled non-zero E-value. This
prevents zero E-values from being scaled above non-zero E-values by effectively
creating two distributions.

Signed-off-by: Ted Gibbons <ted.codes@gmail.com>