Add Consensus Variant WDL Module

The only task left in the ww-leukemia workflow that isn't imported from modules is the "consensus variants" task that looks at all three vcf's and identifies common variants in all three. This will probably be used elsewhere, seems worth it to add it to the modules to fully modularize that workflow.

Already have a Docker image too: https://hub.docker.com/r/getwilds/consensus