N50 - Githubissues

Adds N50 functionality and gets started on a combined csv table.

PR checklist

[x] This comment contains a description of changes (with reason).
[x] If you've fixed a bug or added code that should be tested, add tests!
- [x] 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/genomenote branch on the nf-core/test-datasets repository.
[x] Make sure your code lints (nf-core lint).
[x] 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).