Refactor high-level API by grouping functionality under sub-objects

[confluence]

I've been thinking about how to organise both the code and the user API to break up the huge image and session classes and group related functionality together, and I think that it would be good to split some items into sub-properties on these objects. This would make the code less unwieldy and also simplify some of the API naming. So far some groups I've identified are: the overlay (session), and the raster, contours, vector overlay, and the about-to-be added regions (image). At a later stage we will probably add other functionality that follows the same pattern. Some functionality (e.g. inspecting image geometry and other properties) I consider to be "basic" functionality which can remain accessible directly from the main object. I am considering adding an images group to the session object, for consistency with the regions group I'm planning to add to the image object (the image-opening functions would be grouped under this object).

This would change the user-facing API, so I think we should do it sooner other than later (and it may also affect the e2e tests). My suggestion would make the API look something like this:

session.overlay.set_coordinate_system(...)
session.images.open(...) # maybe?
img.raster.set_colormap(...)
img.contours.set_colormap(...)
img.vectors.set_colormap(...) # simpler than vector_overlay?
# To be added in upcoming region implementation
img.regions.add_rectangular(...)

Does that make sense?

I was considering transparently aliasing these functions to functions on the main session and image objects (for backwards compatibility), but 1) that would be a lot of boilerplate, and 2) it would complicate the naming. So I think we should commit to the backwards-incompatible change now, before the wrapper has been more widely released.

Originally posted by @confluence in #123 (comment)

[confluence]

This was discussed in a comment and on Slack, but didn't have an actual issue.

To do:

• Factor code for simplifying sub-object call_action, etc., out of Image so that it can be reused
• Existing functions should be refactored: in Session: overlay (see refactor AST-overlay related functions  #119); in Image: raster, contours, ???
• Update open feature PRs

[confluence]

This will be resolved by #123.