Add cell-wise density tests once the infrastructure supports setting them

[zhardy-lanl]

@zhardy-lanl can you create an issue to add the test I described earlier: homogeneous medium, fully reflected,
solution saturates to q/sigma_a. next, double the density and the solution should now saturate at half the
previous value. simple enough but would test the change in density.

The test you propose would catch whether or not we missed a spot in the code where we should be multiplying by density but are not. I think it would be equally as useful to have a multi-material test where instead of scaling cross sections we set material densities via whatever API I end up implementing for this.

Originally posted by @zhardy-lanl in #153 (comment)

[ragusa]

The more tests, the merrier. Another test would be to use the exiting options (MakeComined) to multiply a XS (here, a microXS) by a scalar (here, its density), run a simulation, get a PP. In the same lua file, do the same thing but with the new density API and check we get the same value. just a thought.