[WIP] Addition of properties

[talagayev]

Add following rdkit.Chem.rdMolDescriptors:

• CalcNumHeteroatoms for Heavy Atoms
• CalcNumUnspecifiedAtomStereoCenters
• CalcOxidationNumbers
• CalcRadiusOfGyration

[FilippoContessa]

@talagayev could i work on this?

[talagayev]

Hey @FilippoContessa, you can take a look into implementing CalcOxidationNumbers, since the other ones are already done either in a branch or locally on my PC.