Make LayeredIntegral a functor

[lindsayad]

Motivation

LayeredIntegral is very useful for converting multi-dimensional objects into 1D objects for which simple correlations hold. For instance the Dittus-Boelter equation can be used for estimating the Nusselt number for moderate differences between $T_{wall}$ and $T_{\infty}$. $T_{\infty}$ can be computed as a function of axial distance using LayeredAverage aggregation (e.g. averaging the temperature over the pipe cross section).

Design

Make LayeredIntegral inherit from Moose::FunctorBase<Real> (because it is an aggregation we do not want a dense array of derivatives so we do not inherit from ADReal)

Impact

Make LayeredIntegral usable within functor plumbing