Problem:
User enters a chemical formula (e.g. SiO2, Nacl) and optionally a phase. The function returns the material density.

Proposed steps:

• 1. Initialize a structure object (e.g., ase.Atoms?) using the given chemical formula and optional phase information.
• 2. Implement a method (e.g., Atoms.hill_formula) to compute the Hill notation for the chemical formula and attach it to the structure object.
• 3. Retrive matching COD JSON files based on the Hill notation.
• 4. Load the COD JSON files to the structure object, e.g. store it in Atoms.info["cod_entries"].
• 5. Extract and store properties like molar mass, lattice parameters, etc., so that we are ready for computing density.
• 6. Compute density for each COD JSON file: density = (Z * molar_mass) / (volume_cm3 * AVOGADRO_NUMBER).
• 7. Return the average density (per phase, if applicable)

@sbillinge I edited the tasks based on our discussion today. Please take a look and let me know if everything looks good :) I might close some of the current PRs to keep the commiting history clean. Thanks!