[MRG] add a flag to prevent metagenome download from SRA

[ctb]

This PR adds a flag, prevent_sra_download, that when set to True makes the download_sra_wc rule break instead of embarking on expensive SRA downloads.

The output from the rule when it fails is messy but starts with:

** genome-grist is trying to download from SRA for sample SRR5950647,
** but 'prevent_sra_download' is set to true in config.
** Does 'outputs.test/abundtrim/SRR5950647.abundtrim.fq.gz' exist?

Fixes #152