Add the objects magnitudes

[Deuchnord]

Is your feature request related to a problem? Please describe.
An object being risen does not necessarily mean it is visible enough for an observation. Its visibility actually heavily depends on its apparent magnitude.

Describe the solution you'd like
Provide a function that gives, for given date and object, the apparent magnitude of that object from the point of view of the Earth.

The function would look like this:

def get_apparent_magnitude(object: Object, date = date.today()) -> float

The returned value is a float that represents the apparent magnitude of the given object.

Additional context

• We should use Skyfield's planetary_magnitude() function to compute this, and return the average value for the date.
• There are currently some limitations in the algorithm, as explained in the docs of Skyfield. When it returns NaN, we should throw an exception.

[Deuchnord]

Removing from 0.8 milestone and 1.0 project, since the feature seems a little more complicated (see skyfielders/python-skyfield#210).