Labeling single cell rna-seq datasets

### Context

Through my looking for datasets for the tximport subset experiment, we've seen that some of the datasets in refine.bio are single cell RNA-seq and that most of the time (some of the time?)

### Problem or idea

We apparently haven't distinguished bulk rna-seq from single cell rna-seq? (Please correct me on what has been done on this front). It will be necessary to be able to separate single cell from bulk rna-seq in the future.

Although a lot of the initial processing methods will likely be the same or similar for single cell RNA-seq data (salmon and tximport), we will probably need to have an extra normalization step for single cell RNA-seq data. Also, in the future, we will probably not want single cell RNA-seq data and bulk RNA-seq data to be smashed together, so there may need to be some kind of block on that. (Can't think of a reason why users would want or need to compare single to bulk rna-seq. It isn't biologically sensical).

### Solution or next step

1) Single cell rna-seq datasets that are currently in refine.bio should be labeled in some way as to differentiate them from bulk rna-seq data.

2) It should be determined what (if any) single cell rna-seq datasets have been excluded from refine.bio pipeline and whether those datasets should be kept in mind or brought back for when a single cell RNA-seq processing pipeline is established in the future.

### New Issue Checklist

- [x] The title is short and descriptive.
- [x] You have explained the context that led you to write this issue.
- [x] You have reported a problem or idea.
- [x] You have proposed a solution or next step.

Start by reviewing how refine.bio currently represents datasets and how the pipeline handles Salmon and tximport; the issue names no files or tests. Identify current single-cell datasets and any that were excluded from the pipeline, then define the labeling and future handling needed before implementation.

Written by the indexing model from the issue text.