Try to use fabs with std::abs everywhere

This is one of the things one wonders if is possible to do everywhere...

Anyhow as per ATLAS coding guidelines lets try to use std::abs everywhere or at least see where we can not and why ...

The MR is more or less the the result of running

find . -type f -name "*.cxx" | xargs sed -i 's/\<fabs\>/std::abs/g'

We might need to bail out in some cases lets see