General sorting capabilities for VectorPostprocessor CSV output #31913
Description
Motivation
Currently sampler VPPs sort output by either id (can mean different thing depending on the sampler), x, y or z of the samples
This can be insufficient when ids are not consistent in parallel, or points are grouped in groups with the same x y z and the order is insufficiently described
But simply sorting by x then y would let you get a reproducibly-sorted output for a grid, and creating a dummy auxvariable with the desired sampling order would let you sample in any way you want.
@roystgnr came up with the proposed capability in a PR a couple months ago
Note: this is already possible using the MeshDivisions system and sampling using this VPP
https://mooseframework.inl.gov/source/vectorpostprocessors/MeshDivisionFunctorReductionVectorPostprocessor.html
The order is then defined by the order of the mesh division.
However, this might be a little too complicated for users?
Design
Add general sorting logic based on a vector of variables to sort with
Some code is SamplerBase to do that, all samplers benefit
Impact
More flexible postprocessing
CSV output is VERY common and very useful