Calculate Radius of Gyration for a PDB File

### Setup

Radius of gyration is mathematically the root mean square distance of a polymer's atoms from its center of mass, and has its own applications in structural biology and chemistry. It would be great if we could use biopython to calculate it. Specifically, a code like this would be nice:

```python
from Bio.PDB.PDBParser import PDBParser
from Bio.PDB.RG import GyrationRadius

parser = PDBParser()
structure = parser.get_structure("1a8o", "Tests/PDB/1A8O.pdb")
model = structure[0]
rg = GyrationRadius(model)
```

What do you guys think?