Datetime improvements

[philippjfr]

This PR has several related purposes:

• datetime.datetime handled better throughout
• datetime.date handled (addressing Add support for datetime.date #2100)
• Use client-side formatters on bokeh tables to allow data to be shared between plots

[jlstevens]

Looks good.

This reminds me of an issue I wanted to report where the validation for regular sampling didn't seem to work for datetimes after resampling an irregular array onto a regular basis with xarray. That sounds like a fix that might go into a PR though I'll have to come up with a simple reproducible example of the issue unless you already have an idea what I am talking about...

[philippjfr]

This reminds me of an issue I wanted to report where the validation for regular sampling didn't seem to work for datetimes after resampling an irregular array onto a regular basis with xarray.

Please open a separate issue for that.

[jlstevens]

Looks good. Merging.