Add file type selection function

[jashapiro]

In #10 I implemented some steps to convert a channel of project directories to a channel where the processed rds files are represented as a list in the following form:

[project_id, sample_id, [processed_rds1, processed_rds2, ...]]

I expect that this type of function will be generally useful, so it would be nice to have a single function to repeat that performs that transformation. This function would have options for SCE/rds files or Anndata/hdf5 files. We might also have an option to get other files from a project, such as the metadata or bulk data.