Add varnet (https://github.com/skandlab/VarNet)

[kiranchari]

PR checklist

Closes #XXX

• [add varnet somatic variant caller] This comment contains a description of changes (with reason).
• [yes] If you've added a new tool - have you followed the module conventions in the contribution docs
• [tests passed] If necessary, include test data in your PR.
• [done] Remove all TODO statements.
• [yes] Broadcast software version numbers to topic: versions - See version_topics
• [yes] Follow the naming conventions.
• [yes] Follow the parameters requirements.
• [yes] Follow the input/output options guidelines.
• Use BioConda and BioContainers if possible to fulfil software requirements.
• Ensure that the test works with either Docker / Singularity. Conda CI tests can be quite flaky:
  • For modules:
    • [done] nf-core modules test <MODULE> --profile docker