Expand theshholding in ImageFunction to allow assigning multiple regions at the same time

[MengnanLi91]

Motivation

Users want to build mesh directly from experiment images and perform simulations with MOOSE. The current thresholding only support single value and two values assigned to the image data. In fuel simulation, users want to identify multiple regions(>3) and assign different material property for each of them.

Design

vtkImageThreshold can be called multiple times to recursively divide the region into 2, 2^2, 2^3, etc.

Impact

Important for model validation and also help applying MOOSE models to physical scenarios with real geometries other than artificial meshes