Re-add optional vcf input for sample-specific database generation and search

[jonasscheid]

Description of feature

After decision to drop this feature temporarily #248 and to implement this feature we need to

• Add an optional variant column in the input samplesheet
• Copy the epytope module and vcf splitting logic from https://github.com/nf-core/epitopeprediction
• Create a fasta containing mutated protein sequences
• Concat sample-specific fasta with --fasta input
• Generate decoys
• Give comet the concatenated sample-specific fasta

Additionally, make sure if no vcf is provided, that the decoy database is generated only once