Add station visibility "circle" in 2D mode

It would help to be able to see what a ground station's visibility "circle" is on the 2D planisphere. The radius of the circle is computed from the altitude of the satellite, and the 0° visibility (φ = 0 on the picture below).

When projected on the planisphere, the visibility circles are warped (especially at the poles).

The use case is to see quickly if a satellite will be in view of a ground station, using the future trajectory and the stations' visibility circles. It is used every day in operations at CNES and ESA, and can be very useful for mission planning and mission analysis phases as well.