Chemformula

- outsourcing CAS Registry Number management to the [casregnum package](https://github.com/molshape/casregnum) using `CAS` class
- optimizing code
- creating internal parent class `ChemFormulaString`
- complying to [PEP 8 naming conventions](https://www.python.org/dev/peps/pep-0008/#naming-conventions)
- updating [documentation](https://github.com/molshape/ChemFormula#readme)

Update v1.2.5 - adding compare and sort functionality to ChemFormula objects

adding functionality to use the HTML/LaTeX/Unicode property on SumFormula and HillFormula

This update adds a ".Unicode" property to the ChemFormula class that outputs a chemical formula with unicode subscript and superscript numbers.