Allow for more ways to specify density

[jennyfothergill]

Being able to specify density in terms of volume fractions or number density are also important considerations if someone is using a coarse or toy model that doesn't have physical unit connections (yet).

[mikemhenry]

To support this will will have to see if unyt either has a dimensionless unit or if we will just have to make a custom unit to handle something like volume fraction or number density.

[jennyfothergill]

note for later:
unyt does have a dimensionless quantity

import unyt as u

mass = 1 * u.kg
volume = 10 * u.m **3
density = mass/volume

number = 2 * u.dimensionless
number_density = number / volume

results in
0.1 kg/m**3
0.2 dimensionless/m**3

I think we can add some logic to detect different density inputs, (number density, density, maybe even occupied volume?)