Add "aeta" as an acceptable free form cut

aeta performs cuts on |eta| instead of plain eta. gaeta does the same
for generator quantities, and taeta does the same for trigger
quantities.

The "Combined Gen Cuts" ZDefinition now uses these in place of
("eta<2.4", "eta>-2.4").
Use fabs instead of abs

abs is designed for use with integers, fabs is for doubles and floats.
We use fabs now in all places to avoid truncating problems.