MIVOT: making the descripton of the model elements available through the API

As PR #497 is very large, we have decided not to overload it with advanced features (see #532).
This issue (suggested by @gilleslandais) proposes a feature that could be implemented once #497 is merged.

The MIVOT `` element provides access to the `VO-DML` file associated with the referenced model.
These files contain a free text description of each model element. These descriptive items are part of the standard.

It would be valuable to make these descriptions available for the end users.

```python
scs_srv = SCSService(viewer_url)
m_viewer = MivotViewer()
scs_srv.search(
pos=SkyCoord(ra=52.26708 * u.degree, dec=59.94027 * u.degree, frame='icrs'),
radius=0.05
)
)
mivot_instance = m_viewer.instance
while m_viewer.next():
print(mivot_instance.latitude.value)
>>> 59.94033461
print(mivot_instance.latitude.description)
>>> Right Ascension as defined in the Coords model (foo description).
print(mivot_instance.longitude.value)
>>> 52.26722684
print(mivot_instance.longitude.description)
>>> Declination as defined in the Coords model (foo description).

Start by reviewing PR #497 and the related discussion in #532, then trace the Python entry points shown in the example: SCSService, MivotViewer, and mivot_instance. Done means descriptions from the VO-DML model elements are available through attributes such as mivot_instance.latitude.description and longitude.description.

Written by the indexing model from the issue text.