plastid metagene_generate, make_wiggle, psite

[suhrig]

This PR adds plastid with three submodules:

• metagene_generate
• psite
• make_wiggle

These functions are required to compute p-sites from Ribo-seq data.

I have also added test data here: nf-core/test-datasets#1809

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added a new tool - have you followed the module conventions in the contribution docs
• If necessary, include test data in your PR.
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• Ensure that the test works with either Docker / Singularity. Conda CI tests can be quite flaky:
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    • nf-core modules test <MODULE> --profile docker
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