Rename everything to be acorn

This turned out to need a lot of changes. Here is an fairly exhaustive list:

• The top-level directory renamed from gnn4itk_cf -> acorn
• All modules that previously called gnn4itk_cf.X -> acorn
• The CLI command acorn [train, infer, eval] was added, by setting the entrypoint acorn in setup.py, and having that call the main of a new general script acorn/core/entrypoint_stage.py. **Nb: The g4i-[train, infer, eval]` commands are left for backward compatibility
• All example readmes (Example 1, Example 2, Example 3 and the CTD_2023 Example) have been updated from g4i-X to acorn X
• A logo was added to the main readme and the docs/media folder
• The conda environment was renamed to acorn
• The library name in setup.py was renamed from gnn4itk_cf to acorn