ENH: Encode date so that it gets picked up as continuous scale by Auspice

[corneliusroemer]

I think it should be possible to encode the sample date data so that it shows up as a continuous temporal scale.

The dates may need to be formatted in a special way - something like 2022.523 - but that should be doable when exporting.

Advantage: one can quickly see where recent samples were.

I can help with figuring out the format we need from the Auspice side if you're up to this @theosanderson.