Add file type selection function

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.

Start by reviewing the transformation implemented in issue #10, which produces [project_id, sample_id, [processed files]] from project directories. Define what file types and optional metadata or bulk-data selections the function must support, then verify that the resulting project records match the stated form.

Written by the indexing model from the issue text.