Add `date()`/`datetime()` functions to ExprTK suitable for simple data cleaning.

[texodus]

Feature Request

Description of Problem:

Perspective exclusively represents datetime types as POSIX milliseconds values internally, and localizes them to the perspective-viewer's native time zone on display. However, it has some complex rules on how it regards naive datetime types on various sytems, and its not always the case that mis-encoded data sets can be easily normalized before being loaded into a <perspective-viewer>, so it would be nice to be able to use ExprTK to fix datetime columns that are either mis-inferred or flat-out wrong.

Note it would also be great to just document how datetime handling currently works all-around, as the existing docs are buried in the Python details.

Potential Solutions:

A variety of ExprTK functions could be added to make simple correction easy, though this is not an exhaustive list and the specifics of naming/priority are up for debate.

• to_posix()/from_posix() to convert datetime columns explicitly to POSIX float values and back. Note this already basically works with casting except that the type-checked frequently barfs (or allows invalid) types, so this may be as easy as getting proper casting/checking to work here?
• hours()/minutes()/etc time delta creation functions which return POSIX float types, so we can do e.g. datetime(float("date col") + hours(1)) to manually offset timezones.
• apply_dst() to offset only datetime values that fall within DST for the locale.
• Add support for numeric parameters to bucket(), e.g. bucket("time col", '15s'), so that arbitrary buckets can generated without multiple bucket steps.
• Add conversion functions between date and datetime that work correctly. Currently, date casts work in that they cast to their integer representation e.g. the integer 20220628 for Jun 28, 2022.