feat: implement `get_density_from_cloud()` using MP/COD workflow

[yucongalicechen]

Problem

We want to automatically compute sample mass density based on sample composition, if user specified packing fraction.
See #278 for more discussions.

Proposed solution

We could offer other approaches:

1. given a composition, try and get density from COD or Materials Project. For different polymorphs the densities will be different for the same composition, so we would have to handle that ("material-density" not "sample-density").

• Obtain material density from MP API, with the user's API key added to the global config.
• Density = number of atoms/unit cell volume (from composition and cif files), then converted to mass density adjusted by the packing fraction.

3. as a fallback, use a "best effort" atomic density (most tightly packed materials are around 5x10^22 cm^-3)? This would be "magic" which we don't normally like, but if we use this fallback we could give a warning.

[sbillinge]

I was thinking more about this and something like:

if both density and packing fractions are set and raise errors (no change)
if packing fraction is None:
    packing fraction = 1
    get_density_from_material_structure(composition)
  
current workflow for the part when density is passed in.