Add number density to methods in packing.py

[StephMcCallum]

It would be helpful for coarse-grained simulations to pack a box using number density instead of mass density.
A possible solution is to have an option in pack for number density that calculates the box dimensions for you. A solution used in the @cmelab with FlowerMD is to calculate the box dimensions from the given number density: https://github.com/cmelab/flowerMD/blob/main/flowermd/utils/utils.py

[chrisjonesBSU]

I think this would be a great addition. We don't need to follow the example linked above exactly, since that uses unyt, but maybe we change the parameters where necessary to something like mass_density and number_density where the user can specify one or the other (but obviously not both).

Since mBuild assumes nanometers throughout, assuming a number density of particles per cubic nm would be a reasonable approach.