Data offsets and time origin management

* Data offsets can now be removed/restored using methods remove_offset()
  and restore_offset()

* The time origin of a physioset is now stored using a datenum, which
  allows for ms accuracy. The original StartDate interface has been
  kept unchanged. But the StartTime property now returns a datenum
  instead of a string. I will probably fix the latter in a future
  commit.
Backwards compatibility for StartTime

Property StartTime now returns a string representation of the time
origin instead of a datenum. This is done to ensure backwards
compatibility. A datenum representation of the time origin can be
obtained with method get_time_origin()