feat: add "recombinant" qc metric

[ivan-aksamentov]

nextstrain/nextclade#1700

I added this snippet to SARS-CoV-2 dataset (nextstrain/sars-cov-2/wuhan-hu-1/orfs or alias sars-cov-2):

data/nextstrain/sars-cov-2/wuhan-hu-1/orfs/pathogen.json

{
  "qc": {
    "recombinants": {
      "enabled": true,
      "mutationsThreshold": 3,
      "scoreWeight": 100.0
    }
  }
}