_cgs.py contain values that were converted manually from _si or recalculated.
For example:

```
physical_constants['radius'] = (si_consts['radius'][0] * 1.0e2, 'cm', -1)
```

where other values like volume were calculated directly from the radius

```
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).

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.
The _cgs.py file has been completely removed and the values refered
in such file without correspondence in _si have been included in it.
These new constants were extracted from a fact sheet at gsfc.
Renamed _si.py as _constants.py and made the needed changes to the other files to work
made _constants.py file pep8 compilant
solar cycle and oblatness had v as abbreviation, removed and left empty
Merge remote-tracking branch 'upstream/master' into cgsconstant
Merge pull request #968 from dpshelio/cgsconstant

[Review]: Cleaned _cgs.py