Misc tidy up

[muffato]

As I was double-checking #5, I realised that the script was not used any more and could be deleted. This PR does that, and fixes a few minor things: process_single vs process_nompi and the name of script for versions.yml

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 pipeline conventions in the contribution docs
• [ ] Make sure your code lints (nf-core lint).
• [ ] Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• [ ] Usage Documentation in docs/usage.md is updated.
• [ ] Output Documentation in docs/output.md is updated.
• [ ] CHANGELOG.md is updated.
• [ ] README.md is updated (including new tool citations and authors/contributors).

[muffato]

In the end I've decided not to delete the samplesheet script, until it's clear we won't need it at all. And then, the deletion should be exhaustive and include the .csv file itself, references to samplesheet in the documentation, etc.