Added multi chimera steps. porechop, vsearch and blast methods.

[chriswyatt1]

All are optional. I ran the pipeline with all options separately, and they all perform as expected.

We still need a good test dataset to make sure the chimeras as truly detected, and not filtering "good" reads.

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/nanoporemetabarcoding 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).

[chriswyatt1]

For the blast step, it looks for similar hits between the two halves. Searches the top 10 blast hits by default, and checks if they correspond. There is a way to set the number of top blast hits to check.