Dependency Inversion `ChemicalFormula`

[ndaelman-hu]

Atm, ChemicalFormula implements a normalization

1. where it extracts the actual input formula from its containing ModelSystem
2. builds an ase.Atoms representation to produce the formula
3. uses its internal map set all other formulas

Given that ChemicalFormula is a subsection in ModelSsytem, I would move step 2 up to ModelSystem.normalize.
Step 1, is then no longer necessary.