[date parsing] rewrite `numericToCalendar`
[date parsing] rewrite `numericToCalendar`

Rewrite this function to "undo" the `numeric_date` function as found in TreeTime v0.7*. This new functionality correctly restores dates when using `numeric_date` from TreeTime v0.7 (as expected) and restores 5/7 test dates converted using TreeTime 0.6.4.1 (current auspice master gets them all wrong).
nextstrain/auspice,nextstrain/auspice,nextstrain/auspice