Rate limit EBI access

[s-andrews]

I had a problem trying to retrieve all of the files from SRP190015, of which there are around 900!

It actually works to get the ids, albeit that you have to do it in batches, but it won't set up the download links. It gets variably far through the list, but the "loading" icon always stalls and the list is incomplete.

If I'm reading the code right I think it does a separate query to the EBI warehouse API for each accession, with no rate limiting, so either the repeated requests are causing problems, or they're blocking us for spamming.

I tried to add a delay into the code but just succeeded in breaking it. It might be nice to limit the rate at which these requests go out so EBI won't hate me.