Add temporal colouring scale to use YYYY-MM-DD dates

[jameshadfield]

Context
Datasets often have date information additional to sampling date (e.g. symptom onset date, sequencing date, submission date). We have no good way of displaying / interrogating such data. An additional use case is datasets with sampling date but for which internal nodes have no temporal info.

Description
Add a "temporal" colour scale type to the existing 4 types. The values must be either floats (e.g. 2020.123) or YYYY-MM-DD (e.g. "2021-10-31"). We may extend this to allow ambiguity, but this can be left for subsequent work. Auspice will interpret such a value the same way as we currently interpret sampling dates, and generate a colour scale appropriately.

Related issues
#1402 will discuss how filtering UI should behave here. Which branches we show as selected is up for debate, but I propose we treat this the same as filtering to traits where we trace back to the MRCA of selected tips, ignoring internal node annotations.

[jameshadfield]

This was implemented in #1523 however this required all dates to be numeric. I'll leave this issue here but rename it to indicate that we wish to allow YYYY-MM-DD dates

[emmahodcroft]

Allowing both would be good (maybe that is already what's intended) so that current implementations still work! :D