Document example of computing absolute datetime using delta_time

[chuckwondo]

Granule files do not contain an absolute datetime variable/dataset. Rather, they all contain a delta_time (float64) variable, representing the number of seconds since the "GEDI epoch", which is 2018-01-01T00:00:00Z.

For users that want the full datetime of each shot (not simply the date, see #110), document an example in Python and R for computing the absolute datetime for each shot (see computation details at #109).