Get horizon/obstructions from image

[wtgee]

@brendan-o had done some work to automatically find the horizon images from a picture, it would be nice to get that back in.

[wtgee]

This is labelled as a good first issue but is not necessarily super simple.

The basic idea would be to take an image that has a visible obstruction in it and figure out what the altitude and azimuth of that location is given the WCS (or other mount information).

Example obstruction (the Iron Giant!)