img - cell-cell communication

[HelenaLC]

• currently includes a reticulate-based example that runs COMMOT
  (optimal-transport based, links to public resources like CellChatDB etc.)
• this works locally, but not within the BiocBook framework;
  should try whether using basilik solves the issue
• also should add some downstream interpretation/visualization etc.
• potentially include another method(s), though I am not aware of any in R;
  else mention some others (that operate on single-cell level and use spatial info)