Add option to compute maximum of the absolute value to ExtremeValuePPs

[GiudGiud]

Motivation

For some quantities, like the displacement, sometimes it's more about the magnitude of the displacement than whether it's positivie and big or negative and small.
ExtremeValuePP with a max_abs option would be able to compute what we care about in one step rather than 3 currently

Design

Just a new enum option
And a little bit of logic in the PP

Impact

Easier workflow
Empowered users