Simplify CSV export and posfile usage

[LouisLeNezet]

Now the tsv.gz is no more necessary as it is generated directly from the legend file.
All csv export is also simplify with the usage of a dedicated function that is nf-tested (more test could / should be done ?)

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/phaseimpute branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core lint).
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• Output Documentation in docs/output.md is updated.
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).

[atrigila]

Looks good!