Support loop-sentinel with a duration step

[Seddryck]

It should be possible to define a loop-sentinel with a seed and a terminal that are dateTime and a duration for the step.

This duration could be set in month so a loop with a seed of 1-JAN and a terminal to 1-MAR should have three instances that are all a the first day of the month, independently of the count of days in the month. It should also work with days.

The format for duration should be the one defined for TimeSpan for anything with a fixed duration (days, hours, minutes, seconds). For years and months, the format should be "1 year" or "12 months", the "s" at the end is facultative.

[Seddryck]

available in build https://ci.appveyor.com/project/Seddryck/nbi/builds/21296106/artifacts (1.20-beta.48)