Adds support for a range unit, e.g. in 'a month ago', the exact date is TODAY - P1M, but the result is often inexact and has a MONTHS granularity
Renames Temporal to TemporalParse and similarly for Temporal subclasses
Removes nesting of TemporalParse subclasses under TemporalParse object
Extracts out a Period class, separate from PeriodParse
Moves the simpler PeriodParse before the more complex AnchorParse