Add instructions for downloading and pre-processing data for each workload

[fsschneider]

Description

Each workload should have a readme with clear instructions on how (and precisely which) data should be downloaded and pre-processed.
Ideally, this should come in the form of a script, so it can be automated.

See also #104

[runame]

@priyakasimbeg Same here, fixed by #235?

[runame]

@priyakasimbeg Should we close this, remaining issues with the data setup should probably be more specific?

[priyakasimbeg]

Finishing up editing instructions in datasets/README and testing the setup commands. Sent out #510. So I think we can close this.