ancestral: add mutually exclusive group --infer-ambiguous/keep-ambiguous with the latter being default (as it was for vcf, now also for fasta input).
ancestral: add mutually exclusive group --infer-ambiguous/keep-ambiguous with the latter being default (as it was for vcf, now also for fasta input).

nextstrain/augur,blab/nextstrain-augur,nextstrain/augur,nextstrain/augur