Cleaned _cgs.py

contain values that were converted manually from _si or recalculated.

For example:

```python
physical_constants['radius'] = (si_consts['radius'][0] * 1.0e2, 'cm', -1)
```

where other values like volume were calculated directly from the radius

```python
physical_constants['volume'] = (4 / 3. * np.pi *
                                physical_constants['radius'][0] ** 3, 'cm^3',
                                -1)
```

This volume value is slightly different than the one obtained from si (which
has a reference value).

```python
>>> sun.radius ** 3 * np.pi * 4/3.
<Quantity 1.4092725690598589e+27 m3>
>>> sun.constant('volume')
<Quantity 1.4122e+27 m3>
>>> sun.volume - (sun.radius ** 3 * np.pi * 4/3.)
<Quantity 2.92743094014113e+24 m3>
```

Nevertheless, this commits have removed all the values that were available
through si (which can be converted to cgs by just `sun.radius.cgs`) and
commented out these that were not in order to find what to do with them.

Also, it is worth to mention that I did not manage to find a way to import
the values in this file.