devise comprehensive test data for v2/3

[HelenaLC]

• @Artur-man I think we could be close to removing Remotes: completely.
• SpatialData.plot is no-longer used and should be removed (will do in next PR).
• SpatialData.data is used in only few places to obtain, e.g., multi-channel images.
• We should come up with more comprehensive (yet simple) test data for v2 and v3.
• This should include (I think):
  • greyscale, RGB, multi-channel images
  • point, circle, polygon, multi-polygon shapes
  • all transformations for all elements
  • tables for all of labels & shapes
• For long-term maintenance, could ask an agent to revise unit tests to make them access, e.g., element names programmatically. Hopefully that will get us to a test base that is stable upon updating test data.