Implement RoH sub-workflow

[yz533cb]

Modules can be combined into sub-workflows according to:

• shared features
• shared resources
• analyse group

I'm thinking of grouping some of the features into sub-workflows. 'BCFtools-RoH' could be put into an individual sub-workflow, because:

• it requires different resources (process_medium; VCFtools uses process_single)
• it can be followed by a visualisation function roh-viz
• RoH feature itself may require different tools; PLink could be a choice; and some people I interviewed write their own scripts for RoH analysis

[yz533cb]

Tool selection
Use bcftools roh as it integrates directly with the downstream roh-viz tool for generating visualisations.

Subworkflow structure
I'm considering moving the allele frequency calculation via vcftools into this subworkflow, as it outputs a direct input for BCFtools roh.

Test data
RoH is more meaningful when performed on multi-sample VCFs. We can add a small multi-VCF test dataset.