Add metagenomic example

[tavareshugo]

One of the participating institutions used this primer panel: https://www.illumina.com/products/by-type/sequencing-kits/library-prep-kits/viral-surveillance-panel.html

This worked well with the metagenomic sub-pipeline. The pipeline still maps to the reference SARS-CoV-2 genome, as well as performing a de-novo assembly on the reads.