Seqtk add relative sample sets and warning

[nkongenelly]

This code allows subsampling to work with relative numbers too as the initial implementation limited it to absolute number of reads (#50).
seqtk_sample module itself allows passing fractions as the sample_size thus only our schema has been updated here to allow both integers and floats

It also writes a warning to .nextflow.log for every sample that does not reach the selected absolute sample_size

Fixes #60
Resolves #79

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
• If necessary, also make a PR on the nf-core/seqinspector branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core pipelines lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• Check for unexpected warnings in debug mode (nextflow run . -profile debug,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).

[matrulda]

I tried updating the log path to
def log_path = "$launchDir" + "/" + "meta" + "/" + "nextflow.log" (realizing later that the metaDir variable could be used). But the warnings are not written to that log.. I can't find any other nextflow logs in the launchdir or metadir 🤔

[nkongenelly]

@nf-core-bot fix linting please

[nkongenelly]

I tried updating the log path to def log_path = "$launchDir" + "/" + "meta" + "/" + "nextflow.log" (realizing later that the metaDir variable could be used). But the warnings are not written to that log.. I can't find any other nextflow logs in the launchdir or metadir 🤔

Thanks for looking into this. I have now pushed the code that fixes this.

[maxulysse]

@nkongenelly hope you don't mind, I solved the merge conflicts.
I also reworked the test you added, to capture the stdout instead, and used log.warn instead of println.
Changed the view operator for a map operator as well.
And lastly I put the sample name first, to make it more explicit in the logs.