[do not merge] add cores flag

[j23414]

Description of proposed changes

Update documentation since newer version of snakemake requires --cores, matches the zika repo

[j23414]

I see, nextstrain-cli is auto adding --cores=all

• https://github.com/nextstrain/cli/blob/ddcdf9c0cc911204fc90db5b3ff66f52504fae10/nextstrain/cli/command/build.py#L130

[j23414]

In that case, @tsibley is it okay to submit PRs to remove --cores all flags from nextstrain build . statements in our documentation for consistency?

• [x] nextstrain/zika/README.md
• [x] nextstrain/ncov/docs/src/tutorial/example-data.rst
• [x] nextstrain/ncov/docs/src/tutorial/custom-data.rst
• [x] nextstrain/ncov/docs/src/tutorial/genomic-surveillance.rst

@joverlee521 mentioned a good point that some people may be using an earlier version of nextstrain-cli (which would require --cores all). Then @corneliusroemer also brought up a good point that if they are using an earlier version of nextstrain that there will be an error msg telling them to add the --cores all flag back in. Ergo easy to debug for both.

[tsibley]

@j23414 Yep, I'm supportive of removing --cores all from our documented nextstrain build invocations.