[Chemistry] Improve molecule

[yuripourre]

• [ ] Remove bonds structure since it can be inferred by formula
• [ ] Add formula
• [ ] Add molar mass
• [ ] Atoms are closely related to it, maybe they have some attributes in common (update atom class)

Eg:

Concept methane = new Concept ("methane");
methane.setFormula("CH4");
methane.has("molar mass", new Quantity ("16.043", "g*mol^−1);