added new flag --reads_in_memory

[SusiJo]

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] Make sure your code lints (nf-core lint .).
• [x] Documentation in docs is updated
• [x] CHANGELOG.md is updated

[FriederikeHanssen]

Feel free to merge this yourself once you are ready :)