Add comparison methods in AbsoluteDate to improve readability

During the discussion here, the community has decided that some new comparison methods in the class AbsoluteDate and FieldAbsoluteDate would help users of Orekit to write more readable code.

List of new methods to create in class AbsoluteDate and FieldAbsoluteDate (with TimeStamped replaced by FieldTimeStamped for FieldAbsoluteDate):

• isEqualTo(TimeStamped t): true if instance equals t.getDate()
• isCloseTo(TimeStamped t, double tolerance): true if t.getDate() is separated from instance by less than tolerance seconds.
• isBefore(TimeStamped t): true if instance is strictly before t.getDate()
• isAfter(TimeStamped t): true if instance is strictly after t.getDate()
• isBeforeOrEqualTo(TimeStamped t): true if instance is simultaneous or before t.getDate()
• isAfterOrEqualTo(TimeStamped t): true if instance is simultaneous or after t.getDate()
• isBetween(TimeStamped t1, TimeStamped t2): true if instance is strictly between t1.getDate() and t2.getDate(). t1 and t2 can be in any chronological order, but if they represent the same instant this will always return false.
• isBetweenOrEqualTo(TimeStamped t1, TimeStamped t2): true if instance is strictly between t1.getDate() and t2.getDate(). t1 and t2 can be in any chronological order