add a dates module to support a few common processing with dates

[jceresearch]

Typically durations in days/working days between key dates, possibly binning already for histograms/aging.
Working days vs non working days, extra points for holiday calendars.
Possibly all of the about but instead of a against a column against an extraction date provided (or "auto" to pick the latest date in the population on the assumption that it will be effectively the cutoff)
Possibly look at rounding to dates and other related simplifications to make it easy to export to powerbi

[jceresearch]

Added a calendar table for lookups and also a function to truncate dates. A couple of functions do interesting things with dates like sequence and fillna_smart.
To do perhaps something more specific on date processing