Update v1.3.1

Bugfix:

• tailing whitespace removed from string outputs of uncharged formulas

Update v1.3.0

• outsourcing CAS Registry Number management to the casregnum package using CAS class
• optimizing code
• creating internal parent class ChemFormulaString
• complying to PEP 8 naming conventions
• updating documentation

Update v1.2.5

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

Update v1.2.4

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

Update v1.2.3

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