Fix predictions with multiple methods, small improvements

[christopher-mohr]

The problem was the merging process of the method-specific prediction result data frames when using multiple prediction tools. Using the panda method concat seems to solve the problem. However, so far I am not sure why the code didn't work anymore.

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 necessary, also make a PR on the nf-core/epitopeprediction branch on the nf-core/test-datasets repo
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Make sure your code lints (nf-core lint .).
• Documentation in docs is updated
• CHANGELOG.md is updated
• README.md is updated

Learn more about contributing: https://github.com/nf-core/epitopeprediction/tree/master/.github/CONTRIBUTING.md

[skrakau]

Looks good to me