oxidation number

**chemlib==2.2.3**

```
----> 1 Compound('CuSO4').oxidation_numbers()

File /opt/homebrew/lib/python3.10/site-packages/chemlib/chemistry.py:129, in Compound.oxidation_numbers(self)
    126         left.remove(sym)
    128 if len(left) > 1:
--> 129     raise NotImplementedError
    131 ox_nums[left[0]] = int(-current()/self.occurences[left[0]])
    132 return ox_nums

```