feat: Remove ``--theoretical-from-packing`` argument option for muD computation

[yucongalicechen]

Problem

Since we decided to remove the packing_fraction option in compute_mu_from_xraydb in diffpy.utils (diffpy/diffpy.utils#339), it makes sense that we remove this option here as well.

Proposed solution

[sbillinge]

@yucongalicechen I am a little unclear why we want to remove packing fraction here? This UC and the one in diffpy-utils are different. It strikes me that we have two UCs

UC1

1. Till knows mass density
2. till gives lpp composition, mass density, x-ray energy and capillary diameter
3. lpp applies the muD to the data

UC2

1. Jin doesn't know the mass density
2. Jin gives lpp composition, packing fraction, x-ray energy and capillary diameter
3. lpp applies the muD to the data

The UC in diffpy.utils is
du-UC 1:

1. user gives composition and x-ray energy
2. du returns mu for fully dense system.

That would be the desired UC in du, but I am not sure if we have to give it a density or not. If we do, the it would be
du-UC 2:

1. user gives composition, mass-density and x-ray energy
2. du returns mu for fully dense system.

with du-UC 1 as an aspirational future thing.

[yucongalicechen]

Ah okay, this makes sense. I was thinking these UCs are the same. I will close this and the PR then.