rose date: improve CLI

Usage 1: 0 or 1 argument, print time point.
Usage 2: 2 arguments, print time interval.
rose date: improve names and comments
Fix rose date test

Ensure test is not locale sensitive.
Fix `TimePoint.day_of_month` being a `float`, which should be `int`.
isodatetime update

From d4dd6d95adc9a46f892067f2c99e0e510bb9d9d1.
Merge pull request #1332 from matthewrmshin/rose-date-2

rose date: improve CLI