cTiterSub is lacking

[trvrb]

Currently augur titers sub outputs a JSON that has entries for avidity, potency, substitution, titers, but lacks a nodes entry that could be used to decorate the final tree.json via augur export.

This change needs to happen to augur titers sub and then the seasonal build can be updated.

[rneher]

good point. this raises one other issue. The hope was to have the titer sub command be independent of a tree. A nodes field in the json requires a tree. We could either have tree as an optional argument or have a separate rule that attached a subs model to a tree.

[huddlej]

I'd vote for an optional tree argument.