Return pixel positions as single-dimensional columns

This PR addresses #141.

I did not split the input center column if the positions are input `SkyCoord`.  In that case, they are still returned as a column of `SkyCoord` objects, e.g.,:

```
aperture_sum ...                      center_input                     
   MJy / sr   ...                                                       
------------- ... ------------------------------------------------------
 5915.7501418 ... <SkyCoord (Galactic): l=18.384317 deg, b=0.240381 deg>
 1631.4685787 ...     <SkyCoord (Galactic): l=18.38085 deg, b=0.205 deg>
 1854.4486789 ... <SkyCoord (Galactic): l=18.383131 deg, b=0.264819 deg>
```

Saving such a table to an ascii format results in a literal representation:
```
aperture_sum xcenter_pixel ycenter_pixel center_input
5915.750141803959 7.548988470491622 331.64300721143013 "<SkyCoord (Galactic): l=18.384317 deg, b=0.240381 deg>"
1631.468578699616 17.949988574503323 225.5000061500001 "<SkyCoord (Galactic): l=18.38085 deg, b=0.205 deg>"
1854.4486788993265 11.106988506073321 404.95700794457025 "<SkyCoord (Galactic): l=18.383131 deg, b=0.264819 deg>"
```

I would much prefer that the `SkyCoords` column be split into separate columns, e.g., "Galactic l" and "Galactic b" columns of floats with units (e.g. deg).  There does not seem to be a good way to do that with `SkyCoords`.  One could do `skycoords.l` and `skycoords.b`, but that's only valid for Galactic coords.  For ICRS instead, it would require `skycoords.ra` and `skycoords.dec` (and the column names would need to change).  `skycoords.to_string()` strips the units (and gives less numerical precision).

One way would be to loop through the `SkyCoords` with:
```
skycoord[i].frame.represent_as(SphericalRepresentation).lon.value
skycoord[i].frame.represent_as(SphericalRepresentation).lat.value
```
and then assigning the column units with
```
skycoord[0].frame.represent_as(SphericalRepresentation).lon.unit
skycoord[0].frame.represent_as(SphericalRepresentation).lat.unit
```
But I don't think that works with cartesian representations.

Is there a good way to pull the individual coordinate values and units out of a `SkyCoord` object?