Refactoring `SingleCells` to expose more reusable functionality

[shntnu]

Context: https://github.com/cytomining/pycytominer/pull/257#issuecomment-1493329598

Refactoring SingleCells would expose more reusable functionality that CellLocations (and other future classes) could benefit from.

For example, some of these CellLocations methods could be moved into SingleCells or into a new utils module:

Move to a utils module:

• _expanduser
• _parse_s3_path
• _s3_file_exists
• _download_s3

Move to SingleCells:

• _get_single_cell_engine
• _check_single_cell_correctness (needs to be generalized further)
• _get_joined_image_nuclei_tables and _load_single_cell (maybe combine with merge_single_cells)