LUT for factorials, tweaks to input filtering, QUAL < 0 bugfix

And generally, better performance than previous commit.
quick fix to previous commit

Variable casting issue.
fix header, set new GL calculations as default

"Int" should be "Integer" in the VCF header.

The new GL calculations take mapping quality into account by default, so
observation probability is given by (1-MQ)*(1-BQ).