Changed E-values to -log10(E-values)

Now use Decimal package for increased precision with E-values. Changed E-values
to p(E-values) (ie. -log10(E-values)), as is used by OrthoMCL. p(E-values) are
often whole integer values between 3 and 500, so they are stored as
memory-efficient floats after being calculated.

Also changed "_norm" filename extension to "_nrm" so that the file name lengths
will match those of the "_raw" graphs.

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