added tests folder and updates fitness_calculation.R

[BenjaminWehnert1008]

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/deepmutscan 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).

[MaximilianStammnitz]

@BenjaminWehnert1008 please remove the new '\' signs from the fitness_calculation.R file; these would mess with the run

[BenjaminWehnert1008]

For me, locally, it doesn't work without. I think it's because it tries to interpret everything after $ as a Nextflow parameter

Caused by:
  Failed to parse template script (your template may contain an error or be trying to use expressions not currently supported): org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
  /groovy/script/ScriptFEA89ADE780AB746C2B7FA537C9A6C4A: 72: token recognition error at: ''' @ line 72, column 21.
           merged.counts$'wt_aa'[i] <- tmp.wt[merged.counts$pos[i]]