Add alternative assumptions to assume_spherical_screen

[Cadair]

Describe the feature

The assume_spherical_screen context manager provides a very useful way of computing the coordinates of an off-disk point between two different observers.

It would be nice if we could support alternative assumptions to this one, as many people use it as the "defacto" way of fixing off disk data, and having a choice would allow them to make a choice given the data in use.

Things I have thought of or seen mentioned in other places are:

• Assuming a fixed observer for the transforms, this would allow you to ignore observer movement (i.e. between a AIA and HMI image with slightly different observer locations).
• A shortcut for using the Thompson sphere?

Proposed solution

No response