Enable titer substitution model without amino acid translations
Enable titer substitution model without amino acid translations

When amino acid translations are missing, the substitution model needs to calculate
the amino acid mutations on the path between any viruses a different way. This
implementation calculates those differences by traversing the path between two
viruses and their MRCA.

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