Documentation/tutorial

The tutorial contains the following line of code:
```
let sample = Sra.fetch_srr "SRR217304"           (* Fetch a sample from the SRA database *)
```

I cannot find the Sra module anywhere. I may be overlooking it, but has this module been removed in some commit?

Fixing this would be a good opportunity to add tests for the code in the documentation, if such a thing exists in OCaml.