Closes issue-1204: Add methods to retrieve duration from a particular AbsoluteDate in a given TimeUnit
Adds a method to calculate duration from an AbsoluteDate to another in a specified time unit, rounded to the nearest integer and a constructor to build an AbsoluteDate instance from such an offset.
This is useful when exporting or importing epochs from a binary format, for example using an offset from J2000 in nanoseconds.
closes #1204
Edited by Vincent CUCCHIETTI