Add method(s) to transform mapped data between projections

[ortk95]

Inspired by #399 (reply in thread)

It would be useful to add some method(s) to transform mapped data between different projections. E.g. to take a rectangular projection then transform it to orthographic. This would be a logical extension of the observed image -> map transformations that are currently possible. Some potential use cases:

• Merging multiple observations, then projecting to e.g. orthographic - as in Novice user question(s) on GUI #399 (reply in thread)_
• Simulating the view of a planet at a given time (i.e. take a reference basemap image, then transform it to orthographic/stereographic) - this would probably want to be a separate method to the above, as it would make sense to use the image backplanes for this.

I already have some similar code in https://github.com/ortk95/reproject-data that can be used as a starting point for this. Logically, I think this new method(s?) would go in BodyXY - e.g. something like BodyXY.transform_map_projection() and BodyXY.simulate_observation_from_map(). Most (/all?) of the internal projection logic could re-use the existing mapping code in PlanetMapper.

This would also obviously be worth documenting properly, including adding some examples to the tutorials page.