MIVOT: automatically generate SkyCoord instances

[lmichel]

As PR #497 is very large, we have decided not to overload it with advanced features. The reason for this is both to limit the review task, and to ensure that the deep layers are valid before moving on.

This issue proposes a feature that could be implemented once #497 is merged. We could add a module (in the features package) able to automatically generate Astropy@SkyCoord instances from the MIVOT annotations. This would be a 2 step process:

• Check that the mapped models have the data needed to create Astropy@SkyCoord instances.
• Interpreting the model elements as SkyCoord parameters.

The code below illustrates how this new module could be used:

scs_srv = SCSService(vizier_url)
m_viewer = MivotViewer(
        scs_srv.search(
            pos=SkyCoord(ra=52.26708 * easting, dec=59.94027 * easting, frame='icrs'),
            radius=0.05
        )
    )
while m_viewer.next():
    sky_coord = m_viewer.sky_coord()

This approach is meant to be extended to other quantities such as Timeor Photometric data.