problem setting the implicit complement to a material with a density different from that contained in the material libary

[bohmt]

Describe the Bug

If one wants to change the density of the implicit complement from that contained in the material library to some other value using the UW^2 workflow, the documentation indicates that one could use the syntax:
group "mat:Water_comp/rho:0.91_comp" add vol 6 # define implicit complement to be Water at a density of 0.91 g/cm3
However that does not appear to work.

To Reproduce

Use the syntax listed in the documentation to reproduce the problem
group "mat:Water_comp/rho:0.91_comp" add vol 6 # define implicit complement to be Water at a density of 0.91 g/cm3
Scripts to create the material library, CAD file, and mcnp input file for a simple example are available at:
https://uwmadison.box.com/s/g9mzxgapln6hrmjawas3wa0owl43u3no

Expected Behavior

The implicit complement volume is assigned material Water with a density of 0.91 g/cm3

Screenshots or Code Snippets

_bohm@manitou:~/neep/testuw2dagmcnp62$ ~/dagmc_bld/DAGMC/bin/uwuw_preproc shieldblock704_zip.h5m -v -l /home/bohm/neep/testuw2dagmcnp62/testmatlLib2.h5m
Loading file shieldblock704_zip.h5m
Initializing the GeomQueryTool...
Using faceting tolerance: 0.001
Making materiral group name mat:Iron
Making materiral group name mat:Water
Making materiral group name mat:Vacuum
Making materiral group name mat:Graveyard
Making materiral group name mat:Water/rho0.91_comp
Materials Present, :
Iron, Water, lowdensityWater,
Making new material with name : mat:Iron
with fluka_name: IRON
Making new material with name : mat:Water
with fluka_name: WATER
Making new material with name : mat:Water
with fluka_name: WATER1
writing material, mat:Ironwriting material, IRON to file shieldblock704_zip.h5m
writing material, mat:Waterwriting material, WATER to file shieldblock704_zip.h5m
writing material, mat:Waterwriting material, WATER1 to file shieldblock704_zip.h5m

bohm@manitou:~/neep/testuw2dagmcnp62$ dag-mcnp62 n=shieldblock704 gcad=shieldblock704_zip.h5m lcad=shieldblock704.glist
mcnp ver=6 , ld=06/03/20 01/11/21 09:49:17
Loading file shieldblock704_zip.h5m
Initializing the GeomQueryTool...
Using faceting tolerance: 0.001
Building OBB Tree...
This problem is using DAGMC version 3.100 w/ DagMC r 0
Going to write an lcad file = shieldblock704.glist
Material with name mat:Water/rho0.91_comp not found
In the material library_

Please complete the following information regarding your system:

• OS: ubuntu 18.04 LTS
• MOAB Version: 5.1.0
• Physics codes versions installed with: MCNP 6.2

Additional Context

Assigning the implicit complement to an existing material and density contained in the material library works using:
group "mat:Water_comp" add vol 6 # defines implicit complement to be Water at the density specified in the material library.