Document expected output for cell type annotation modules

[allyhawkins]

In #172, we are adding a module that will export annotations from OpenScPCA-nf to a JSON file that will be accessible in a public bucket. As input to that module we require the cell type output tuple to be [project id, sample id, tsv files, annotation metadata] where annotation_metadata is a named tuple with module_name, annotation_column, and ontology_column.

For any modules that export cell type annotations, they should have this tuple and then add to the input channel of the export_annotations process in the main workflow. This will all need to be documented and added to the instructions for porting modules.