Let particle managers move particles around within cells to reduce particle clustering

[JarettBakerDunn]

This pull request lets particle managers move particles around within cells in order to avoid excessive clustering within cells.

The idea behind it is that while some previous additions to the particle manager allowed the particle manager to add and remove particles according to particle density when there were too many or too few particles within cells, there could be cases where particles could cluster within cells because of the underlying flow and not because of the particle per cell bounds.

There are 4 commits in this pull request:

add ParticleHistogram class: this commit moves some code from the ParticleDistributionScore postprocessor into the new ParticleHistogram class. The ParticleHistogram class now contains the code which quantifies the degree to which particles are clustered within a cell.

let particle manager redistribute particles within cells: This commit adds the functions and parameters needed for the particle manager to use the new ParticleHistogram class to measure the level of clustering in each cell, and if the particles in that cell are clustered enough, to use the ParticlePDF class to find the most clustered particle to move and the least particle-dense location in the cell to move it to.

make ParticleDistributionScore use ParticleHistogram: in order to avoid duplicating code, I made the ParticleDistributionScore postprocessor create an instance of ParticleHistogram to score each cell

The last commit adds a test case and the documentation. I also added a benchmark which demonstrates that the new feature is working, but I am not sure if this is ideal because it is not a physically accurate setup, just a simple model that causes the particle to cluster with prescribed velocity.